Dang if I know, But I can end the Stainless vs Nickle debate...
A magnet is your friend..! check it with a small magnet... if it draws to the gun hard, it is plated... if it is non-magnetic or a lighter pull, it is Stainless. 410 series stainless is still magnetic, but the attraction will be weaker..
Just to make it clear as mud.....
Chemical Composition Chart Stainless Steel
UNS# ALLOY C Mn Cr Mo Ni Fe Si P S Al Cu Zn OTHER
S30400 SST-304 .08 2 18-20 -- 8-10.5 BAL 1 .045 .03 -- -- -- --
S30403 SST-304L .03 2 18-20 -- 8-12 BAL 1 .045 .03 -- -- -- --
S31600 SST-316 .08 2 16-18 2-3 10-14 BAL 1 .045 .03 -- -- -- --
S31603 SST-316L .03 2 16-18 2-3 10-14 BAL 1 .045 .03 -- -- -- --
S32100 SST-321 .08 2 17-19 -- 9-12 BAL 1 .045 .03 -- -- -- Ti 5 x C Min.
N08330 SST-330 .08 2 17-20 -- 34-37 BAL .75-1.5 .03 .03 -- 1 -- --
S34700 SST-347 .08 2 17-19 .75 9-13 BAL 1 .045 .03% -- .50 -- --
S41000 SST-410 .15 1 11.5-13.5 -- -- BAL 1 .045 .03 -- -- -- --
S43000 SST-430 .12 1 16-18 -- -- BAL 1 .045 .03 -- -- --
The screw you need is widely available, Brownells, MIdwayUSA, Numeric Arms... and probably at your local gun shop.. should be able to order it in the finish you need..