What material is currently used in the recievers on the 336, 8620 or 4140 ? I'm wanting to color-caseharden my pre-safety 336 and I don't want to grind on it to find out what it is. If it's 8620 is the trigger-plate (lower-tang) the same? I'd like to know, I'm thinking of making some straight-grip lower-tangs and I want to make them from the same material. THANKS!