Go to
http://stainlessbits.com/link12.html this site , download the list & then print out a copy & keep it in the workshop.
As for different part numbers for the same bolt, many of them have different thread lengths which will naturally give them a different part number.
Polishing the head will also mean they have a different part number.
As for using Allen headed cap screws you should always use at least 1 if not 2 washers under them.
Otherwise you will eventually destroy your covers.
BSA used fillister heads particularly because they are the largest standard head.
This was to spread the clamping force over a large area under the heads so the heads did not either collapse the clamping zone back into the hole or dig a deep ditich and distort the bottom of the hole.
I have seen some covers where the DPO has actually managed to drive the heads totally through the cover.
An Allen head has 1/3 the area under it as a Fillister head.