AFAIK when delegates of XXV congress CPSU arrive to Moscow, they was given bag(s) with goodies, included specially made office supplies (pen(s) notebooks est.), they was given special badges, souvenirs, brochures, they was given passes to theaters, and they was given cameras with commemorative stamp. Mostly Zenit E, but by now we are discovered Zorki 10 with such stamp. What very well can tell that not every delegate was equal. Possibly delegates was divided by ranks and every rank have they set of goodies?
It would be very interesting to know how CPSS divided "equal" delegates and who got what.

Possible that other goody-bags was given as bonuses or as "tips" to organizers and servants of the congress and included commemorative cameras of different types?
Also possible that KMZ had trouble to came up with needed quantity of Zenit E (5000 cameras is a huge quantity, probably month of production of entire assembly line) so they added other types of the cameras.
I doubted cameras with such stamp was ever sold in retail.