you can always take the rail to homedepot and see which set screw will fit.
my earlier rail did not have a hole for the set screw but my newer gen 3 & 4 does.
for those who use the rail for their FSN , you will find that due to the use of bluing the rear sight block will be a bit narrower then the painted blocks so your rail will be loose . i use an adhesive aluminium foil , i cut it to the shape on each insides (see pen below).
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