So it's possible to make a collection of Soviet Ukraine for 1918, 1919 and 1920 but very much made up of bits and pieces from the fairly short or short-ish periods during which the Reds controlled now this, now that bit of territory. It's not always easy to identify the material but there are some useful Check List points:
- the UNR never had a Free Post so Free Post items are always Bolshevik
- UNR and RSFSR Tariffs are different
- though there are periods in which mail passed back and forth between the RSFSR and the UNR (for much of 1918, notably) at other periods this is not so and any mail going from Ukraine to Moscow or Petrograd or other obvious RSFSR areas will be Soviet mail
- in the UNR, unoverprinted Imperial stamps were invalid from 1 October 1918. But unoverprinted Imperial stamps were the mainstay of RSFSR mail throughout the 1918 - 1920 period.
One notable aspect of Soviet mail in 1920 and through into 1921 - by which time the UNR Government was in exile - is that it uses Ukrainian General Issue and Trident overprinted stamps, but with the General Issue stamps and the Imperial low values up to 20 kopecks revalued x 100 in accordance with the March 1920 RSFSR revaluation.
What I don't know is whether there was a formal invalidation of Trident stamps and,if so, when. I have Tridents non-philatelically used on Soviet mail as late as May 1921 - see the example below. An "obvious" date for invalidation would be August 1921 when the new Arts and Industry stamps, and new Tariffs, came into use.