I have a number of examples of the three values used. On two of the 5 kopeck values there are four numerals. 7500 reading upwards and on another stamp there are four numerals beginning with a 7 but the rest are not distinct but reading downwards.
Can someone explain the significance of these numerals and why they are different ways up?
From your description your stamps could well be the RSFSR Local Issues from Kiev, 1922. There were several overprints made to fit both the increase of the postal rates and the lack of standard stamps. Those issues are known with overprints reading upward and downward (as well as diagonal). They were used as postal stamps (rare on cover). Beside 7500r, values of 8000 and 15000 have been overprinted and used.