I believe, according to the parts information available. That is a PCI connection for a riser expansion module which allows for a PCI and PCIeX1 card connection. Here is a picture of the expansion module in question as well as the picture of the motherboard so you can try and match them up. I doubt that you can use a PCIeX16 card in there as the bandwidth would have to go through the normal PCI lane to get to the rest of the system.
 
 
 

If what is say is true, and I have no reason doubt you, the Slot is not a PCIe x16.  It cannot be called such unless it is fully compliant with the PCIe specifications.  That mean an x1, x4, x8, or x16 can be plugged into it and fully function.   All Poooo to HP, they lie. 

scaryone
I'll have to update the picture when I get a chance. I don't have a moment right now. The board itself looks like just a PCI slot for the riser board in either case. It won't give you the PCIeX16 speeds since you are having to go through an additional pathway to get at the CPU itself. Since the CPU and PCIe slots work in conjunction with each other to attain the speeds I really have serious doubts you can even get close to PCIe_x16 speeds using an additional riser.

I edited my post above about your first picture, unless you've recently fixed it.
NO it's not just a PCI 32 slot. it has 82-pins on each side.

Here's a shot of the HP PCI 32 Riser against a PCIe x16 riser extender, same length. same number of pins.  ... I tried to attach a second pix of the two on-top of each other to get a better idea of the actual pin for pin connections, but this forum software isn't showing the second attachment for me to add to this post

KenMcC
If what is say is true, and I have no reason doubt you, the Slot is not a PCIe x16.  It cannot be called such unless it is fully compliant with the PCIe specifications.  That mean an x1, x4, x8, or x16 can be plugged into it and fully function.   All Poooo to HP, they lie. 

It's true it's not yet compliant with standard PCIe x16 slots, but that's because HP didn't wire it to standards of the slot... BUT, if I can get the wiring of the way it is on the motherboard, I can create a riser that adapts the wiring to standards of a PCIe x16 slot.
 
I have resources to make printed circuit board designs and etch another riser card custom made for the T5730. 
First I have to make sure, as you say, it has all the supported lines there.
All my other modification to the board work... the new 64-bit Turion X2 Dual-Core CPU @ 2.4Ghz
The 320 GB Hard Drive, a higher wattage 12vdc 16 amp PSU (192w) + the 2GB of RAM, and there's another socket Board Trace Pattern on the mobo for a second RAM module, to allow Dual Channel RAM operation, maybe to 8GB, but I'm only using a 32-bit OS for now... and would just go to 4GB total RAM... so the RAM socket is next if I can't find info on the PCIe socket of the HP board.
 
I had it working with Windows7 64 Pro, but didn't like the OS for what I was doing with it.
There's even a board pattern on the botton of the mobo, for a CF Slot socket :)
 
Oh and I have the datasheet books on the chipset, and it says it supports a PCIe x16 slot, along with many other things HP never mentioned, like supporting HDMI output and TMDS output.
 
Being HP didn't really make these devices, I suspect they "watered it down" from the company designing it, and customized it for their own products sales... each Thin Client, in this basic series, is really the same motherboard, with small changes added or omitted, from the original design.

scaryone

Have you taken a look to see if it conforms to the PCI-X slot designs? It is possible they are using that for the connections. I did happen to update the picture and changed the location of the image.

 
I understand what you're saying, ... they may have just put a 174-pin slot there to bring out selected lines to their riser card they made, BUT, and that's a BIG BUT..., (only one T, :) ... I don't think they used an expensive PCIe x16 socket, just to have the ability to connect to different sections of the motherboard... not when the data sheet book says the chipset supports a PCIe x16 socket. (this is why I'd like to see the actual motherboards schematic)
 
I think the T5740 or T5745 has the same board, and supports a PCIe x16 , and has a different Graphics Chip on-board, but that's the only difference ... the T574x has an option to purchase with a Dual-Core CPU, and I was able to replace the Sempron single core 1GHz, with the Turion 64 X2 Dual-Core 2.4GHz CPU, in the T5730, but HP never said it would work, so ... there ya go ... and chances are, the "PCIe socket" is wired differently, so HP can control the features each of their Thin Clients will have, using the same basic motherboards.

It's been done many times before.  ... the name brand company contracts a china or japan company making a basic design, to customize it for their own use and sales, but uses the same main board with parts added or removed for each different product the name brand company wants to sell, and each different name brand company is offered the same basic design to have customized for their own use.
 
D-Link did it with the WiFi phone DPH-541 and 540. 
It's a basic design made by Broadcom, and re-worked for the name brand company to sell as their own.
 
Broadcom also offered a Video Phone, with most of the basic same chipset, except for the updated Video Phone sections in the newer chipset.
 
I feel it in my bones ... this is the full PCIe x16 slot... with different wiring custom made by the original company, making it for HP.
BTW- here's my Desktop showing the CPU-Z and CPUID HW Monitor programs with the Dual-Core processor in and working, and a very cool temp at idle... as proof I have it working.
 

Interesting and not all that unusual when a major reseller decides to change things up a bit. I would believe that you may be right but as far as having the schematics just out there I doubt it is available outside someone not internal with HP. I don't happen to know anyone in there anymore so my resources are very limited in that regard since most of who I knew worked for Compaq a the time prior to the official change of branding.
 
Good luck finding it. If I do happen across it in my searches I'll post here again.