That's soo wierd, I e actually talked with him over PM's over at the other site. Cool dude.
Yeah the 13.5's shoulder is right at the end of the rail. Looks like his chassis is the same. I'll shoot him am PM. I am curious now.
Gratuitous 13.5 pic
https://image.ibb.co/czcm20/20181114-173228.jpg
https://image.ibb.co/cEpzyV/20181111-201624.jpg