I am curious about this also. Hope you get a answer.

 

I own a pt638 and have just purchased a PT 138 magazine. right away I can tell the very base of the magazine is different but everything else looks almost exactly the same. I will attempt to make minor modifications to the bottom base plate of the magazine of the 138. I will update as goes

 

Moved to the Millennium Pro section.

 

Yes they actually do but u need to remove the base of the mag