Welcome @corinne94, to the community!
The ST6 family has unfortunately been extinct for a very long time, which is why detailed information is no longer available. However, you can still find the ordering information in the last data sheet, section 11.5, according to which the C after T20 stands for a product definition change. So if you can still find ST62T20M6 somewhere, it must be an extremely old or a fake device, as the B and C variants were introduced before 1996.
You will probably only make the seller happy if you buy the ST62T20M6 on a trial basis. I wouldn't buy such old devices, especially as you can't be sure that they work at all or that they are still empty and programmable.
I am extremely sorry not to be able to give you a more pleasant answer.
Good luck!
/Peter
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