It's just the Russians having a bit of fun with us....... Seriously, counter stamps of this nature are quite common and fun to find.
These were counter stamped after leaving the mint (PMD) as souvenirs. No added value but they are neat.
I have literally dozens of the Kennedy c/s cents from roll searches. They seem to have been a big thing back in the early 1970s. There are even different varieties of Kennedy's portrait on some of them.