A bit late maybe, still some comments:
There had been quality issues with the first versions of the ZWO OAG, that were actually design flaws. They added more/better fixation screws, and with that the OAG (IMO) was rock solid. I've used it with my Stowaway and the ASI1600 for 1.5-2 years.
I doubt however that the old one can be properly used with the large new EFW and the 2600, definitely not with a non-mini camera. You can "fix" this by moving the OAG further away from the EFW using a spacer in between (at least if you have more than 55mm BF requirement). But then the focal plane also gets (far) away from the hole in the prism stalk, and you will get (very) strong vignetting. Also not desirable.
The M68 OAGs helical indeed cannot be removed, so no real way to attach the non-mini cameras, unless far away from the prism. Again, this could be compensated by moving the OAG further away from the camera. Maybe the vignetting would be less severe with the larger prism of the OAG-L....
Your comment regarding the revised original version is exactly the information I was looking for in my initial post. I had read a good many negative comments, but even though I suspected it, I didn't find any direct reference to a older and newer version. I did not want to build my new image train around an OAG I was going to have problems with.
After posting and digging even deeper, I found the info on the revised version and ordered one. As you can see, the full size 290MM fits just fine with the 2600MM and new 36mm EFW. Of course the OAG position needs to be fine tuned to the EFW, or clearance disappears rapidly! And for disclosure, I have not yet focused both camera's... plan to later today when it cools down.
Build quality seems very good. Other than the small prism hole, which is odd, the 3 lock down screws could be longer so you could get a good finger hold on at least 2 that would then extend beyond the FW. Also, on the scope side, the M48 threads go all the way through to the stem without a hard stop, so your adapter's threads may be longer than the space available. In this pic, the flattner bottoms out on the shoulder of the flattner. On my 120mm scope, the threads on its flattner are longer, so it bottoms out by hitting the prism stem. This resulted in using up an additional 1.5mm of backfocus (OAG edge to flattner shoulder) which I did not anticipate on paper. So that is something to keep in mind.