HK bipod, finally

RobertRTG has been promising Pakistani-made HK91 bipods for some time.  They finally arrived, and I bought one, because I am not going to spend as much on a gen-yoo-wine Kraut bipod as I did for the rest of the damned rifle. 

Eagerly I lifted it to the forearm of my Springer/Greek HK91 to find that the bipod just would not stick on that little device there. 

Back to the interwebs, to find that Springer had milled off half of that little device there, along with slapping on a perverted thumbhole stock.  I bought that rifle new in 19-mumblety-mumble when certain cosmetic features were disfavored. 

New forearms cost as much, almost, as the bipod.  The value proposition of the affordable POF bipod was dissolving before my eyes. 

Until I found a scrap fender washer of about the right dimensions, held it with a vise, and carved away all the metal that looked like it would not help hold that bipod on. 

Then I wire-feed welded that chunk on the forearm, about where the necessary parts were ground off.  The bipod now hangs there, and bears a little weight.  I need some pointers on what sort of spring was riveted inside the handguard as a catch to let the bipod come back off.  Any ideas or diagrams, dear readers?