I passed your question along to a colleague of mine. He wrote:
I designed the original 12 ga. with a blind top loading mag that used the 110 stock die/mold with inserts and a new top slide for the action.
The stock was the most expensive tooling @ about 20 grand v/s 140 for a stock mold.
The receiver was investment cast including the top of the mag well and feed lips. Bottom was molded in the stock.
It had a three lug front lockup with 60 degree rotation. It used a 110 trigger.
After I retired it was changed to a removable mag design and a 20 Gauge added.
Regards, C