I found this at Glockmeister...
* Indicates ejector and mag change.
Receiver------------Upper/s
G17.....................G17,17L,34
G17L...................G17,17L,34
G19.....................G19
G20.....................G20,G21
G21.....................G20,21
G22.....................G17*,17L*,22,24,31,34*,35
G23.....................G19*,23,32
G24.....................G17*,17L*,22,24,31,34*,35
G26.....................G26,27*,33*
G27.....................G26*,27,33
G29.....................G29,30*
G30.....................G29*,G30
G31.....................G17*,17L*,22,24,31,34*,35
G32.....................G19*,23,32
G33.....................G26*,27,33
G34.....................G17,17L,34
G35.....................G17*,17L*,22,24,31,34*,35
I want to go with a 22 upper.
Yeah, I would have to find someone wanting to trade or sell one to make it worth it.