i have notice that the soviet 1961 series have on the identification number different size leters. however, on 2 diferent 5 roubles i have notice a GG identical size and a sa( cA)different size. it is a error?

not an error, just a different variety. I have seen 3 types so far.

two capitals (AB)
upper & lower case (Ab)
lower & upper case (bA)

i don't know the reasons behind the different types, but there's probably an interesting story behind it.

The 1961 5 Rubles notes were printed for 30 years, until 1991 when a shortlived new series came out. I got a kick out of spending money that seemed so old, but had probably rolled off the printer in the last couple of years.