What Marlin did do was modify the thickness of the wood at the head of the stock
"L.C. Smith stocks have been re-designed for still greater strength, shaped to hold the frame solidly, with special attention to tight, sure mechanical action. All stocks of selected, seasoned walnut..."
Please see the "Are L.C. Smith stocks predisposed to cracks" in the Maintenance and Restoration FAQ
Marlin era 20g guns of course had 2 3/4" chambers. I haven't seen a Marlin era hang tag, but Hunter Arms tags listed 2 1/4 dram 7/8 oz. shot for 20g, and that is what I'd use, unless the head of the stock has been glasbedded.