What you have is correct, the rear hole is shared by the safety/trigger spring screw and requires a very short screw to mount the sight - the screw should be just short enough to tighten so there will be as many threads as possible left to hold it.
The screws are 8-40's and my local ACE Hardware started stocking sets of gun screws that include several sizes of these in a couple head styles that include the ones for a tang sight, prices vary by length but are all under $1 each. These screws look like the same set Brownell's sells which look like the the same as the ones offered on eBay for about $5 a set of two with added shipping.
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