Beware of extremely tight-margined stamps offered as 312/313.

The margins on these issues are usually generous, and the earlier issues are worth 5-20 times the later ones. Therefore there is a temptation to reperforate a large margined 479/480 to fake a 312/313. The arrows below are the same length on both stamps. The red ones prove the design is the same size on both pictures and the green ones show how much smaller the left stamp is overall. The stamp on the left is likely a fake.

How do you tell an undoctored 312/313 from their undoctored counterparts?

Simply count the perfs along the top and/or bottom - 10 = perf 10 479-80, while 12 = perf 12 312-3.
You would be amazed at how many 479-80 are sold as 312-3 and
the prices that they realize even when they are not reperfed!

The stamp on the left below has been reperfed, note the narrow margins