Originally Posted by SSGN_Doc
The Glock .40 frames have aditional pins and reenforcement the the 9mm frames do not have. this is the reason that you can go .40 to 9mm but not the other way around.
On the older 2nd gen guns that would be correct---the 9mm guns had a
single pin locking block and the 40 guns had a 2 pin locking block.
Starting with the 3rd gen guns (at least 10 years ago) they all have
2 pin locking blocks.
Currently the lower frames on 9mm and 40 S&W Glocks are identical. The
only part on the lower that is different is the ejector, which is not actually
a part of the frame.