The 10-rd Glock 17 magazines I have are a completely different body than the 17-rd versions. I've never gutted one but first suspicion is they use a thicker body and have a limited internal volume. They are definitely not the standard body with a block in place. The 10th round goes in hard, they seem to be designed so you can't gorilla #11 in there accidentally. And they are 'ell for stout; I'll keep using them for practicing reload drills over concrete and spare the "high caps" the abuse.
(Yes, I've had my Glock 17 since the AWB was in place. I also have 2 police-trade in "pre-ban" mags I think I paid $45-50ea 11-12 years ago. They've never given me a lick of trouble either.)
NRA Master, Highpower Rifle, Across-the-Course
NRA Expert, Highpower Rifle, Mid-Range Prone