I'm not too confident of this 'engagement'...
The takedown lever has a cut in it to clear the curved bottom of the barrel extension...But it comes nowhere near it...
I'm trying to measure the gap between the lever and the bottom of the lug...But I know from the pic that it ain't much...
The lug now sticks down .085". I stuck a piece of masking tape to the front of the lug and 'painted' the takedown lever rear face with a magic marker...Assembled it and rapped the lug against the takedown lever a few times...The first pic shows the amount of engagement...About halfway...
Second pic shows amount of clearance between the barrel extension lug and frame cut...Third pic shows amount of clearance between takedown lever and frame broached cut...
Building up the barrel extension lug and/or the takedown lever isn't all that practical...Not without milling/broaching the frame (to allow more engagement surface)...
I'm going to check my 1900AE tomorrow to see how much 'engagement' surface it has...