You may find this thread to be of interest: NitroExpress MS Forend Pin

Inch --- Millimeter --- # - Description
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1.312 --- 33.32 --- 01) Overall Length

0.073 --- 01.86 --- 02) Thickness of head*

0.079 --- 02.00 --- 03) Head to Flange

0.032 --- 00.82 --- 04) Flange Thickness

0.187 --- 04.75 --- 05) Top of Head to Bottom of Flange

1.063 --- 27.00 --- 06) Flange to Radius at Bottom of Pin

0.445 --- 11.30 --- 07) Diameter of Head

0.251 --- 06.38 --- 08) Diameter of Flange

0.185 --- 04.70 --- 09) Diameter at Inside of Radius Between Head and Flange

Diameter of Pin:
0.197 --- 05.00 --- 10) At Flange

0.179 --- 04.54 --- 11) At 13.5mm Below Flange (mid-point to bottom radius)

0.165 --- 04.20 --- 12) At Start of Radius at Bottom of Pin

0.850 --- 21.60 --- 13) Center of Spring Pin to Flange

0.874 --- 22.20 --- 14) Bottom of Cut for Spring (to flange)

0.064 --- 01.62 --- 15) Thickness of Spring

0.065 --- 01.64 --- 16) Width of Slot for Spring

0.106 --- 02.70 --- 17) Notch in Spring (Catch)

0.221 --- 05.62 --- 18) Width of 'Push Pin' Face (Knurled)

0.092 --- 02.34 --- 19) Thickness of #18 at center (ends taper upward in a 'moon' shape)

*02) Underside of Head of Pin tapers downward toward center - thickness measured at edge where knurl begins. This is why #s 2+3+4 = less than #5

The plates mounted to the stock have holes of the following size for the pin to pass through (from right to left); Right Side - 5mm Left Side - 4.4mm .

Images from 1939 Stoeger Catalog: