What are the differences between the GSGs marked with the ATI logo and the ones marked H&K? It is marked H&K on the upper.  Do the parts interchange?  Does the H&K stock differ?   I am looking to change he stocks, have the telescopeing stock want a solid one/standard.  What can you tell me.

I believe they are in no way related. The ATI marked ones are made by German Sport Guns and imported by ATI. The H&K marked one is made by Walther for H&K and imported by Umarex. I'm not sure what parts if any can be interchanged.

OK, If not interchangeable, Where can I purchase a solid stock?

I've got a solid stock if you wanna trade if you have a collapsible stock

Sent you a message on the srock.

I bought an actual MP5 stock and put it on my ATI GSG5 when the original plastic stock broke.  It is eleventy-thousand percent better, rock solid.  I had to dremel out the mounting holes very slightly, but other than that it was an easy fitting.

I believe this is the guy I bought mine from:

http://www.gunbroker.com/Auction/ViewItem.aspx?Item=363919712

www.hotgunparts.com also has a high quality stock that fits perfect right out of the box.
Nice heavy duty stock.  Fits snug and feels way better than the stock plastic one
Even takes the Hk pin replacement with no modifications/drilling whatsoever.

http://www.hotgunparts.com/servlet/the-41/High-Quality-Standard-Stock/Detail

http://i60.photobucket.com/albums/h37/fuzzystump/627f83dc-82ca-44e6-a806-a76a3518f5a0_zps365cf4b8.jpg

Thanks for the picts.  I have found that the GSG stock will not fit the H&K .22 MP5a.  I am still looking for the correct solid stock.

I misunderstood.. thought you were looking for GSG Stock.  I actually own a GSG-522 and an H&K MP5, and have replaced the stocks on both.

Here's the link to where I bought my solid stock for the H&K.  It's a little pricey, but I wanted it in place of the telescoping one so I was willing to pay for it, I'm extremely happy with it.
It's actually for a different model so you'll need to use a dremel tool to sand down the inside ribs in the stock a little to make it fit, but it's no biggie.  Took me less than 10 minutes to get mine to slide on and off nicely.
Seems like the one for the 22 version is permanently out of stock
I actually have a pic of the H&K and GSG side by side on the most recent page in the pics thread if you want to see how the weapon looks with the solid stock.

http://www.hkparts.net/shop/pc/HK-MP5-A2-Fixed-Stock-German-237p169.htm

Good luck!  Let me know how you make out!