Grabbed a copy of FreeCAD and went to work today teaching myself from scratch.
I couldn’t do any old project so I drew up a trunion for an AR-180B.
I reverse engineered the dimensions from my original upper I found it to be a lot of fun.
Just a quick picture.
The hole takes a modified AR-15 barrel extension, just like the original 180B.
I am still not sure how the original was retained other than a press fit.