Russian Occupation of Latvia, 1919

Stamp collectors often have a pocket notebook containing their Wants List - carry it everywhere and you will never have to hesitate over a possible purchase, "I may already have that".
Postal historians likewise carry around lists of tariffs or cancellations.
Not so often seen are lists of dates. If I was better organised, I would carry a notebook containing key dates for the Russian revolutionary period 1917 - 1923. It would be enormously complicated: I think it was Kyiv (Kiev) which changed hands twelve times in that period.
It was also a period in which the calendar changed, but not everywhere at the same time, so that you can sometimes find letters which arrived before they were despatched.
Recently, I realised that it is quite easy to miss the Russian Occupation of Latvia in the first half of 1919. The Bolsheviks entered from the east through the rail junction of Dvinsk (Daugavpils) at the very end of 1918 and captured Riga in early January, holding it until May.
You occasionally see philatelic items from the Occupation, most commonly blank envelopes cancelled with Latin "Riga" over 35 kopeck and / or 70 kopeck Kerensky Chainbreaker stamps (designed by the Latvian Zarins[h] who had earlier worked on the Romanov set and later designed Latvian stamps as well as the unissued stamps of the Belarus National Republic ....An interesting history).
Commercial mail is scarce and boringly franked by Imperial arms stamps. So unless you note the cancellation and its date, you won't realise that these are Occupation usages.
Recently, I looked despairingly at two 1919 Money Transfer Forms where the adhesives (including Postal Savings Bank stamps)had been clipped with unusual brutality as an anti-fraud measure. Then I cheered up: both the cards were going from Russia to Riga in the Spring of 1919, with arrival cancellations and a Riga post office seal. I had never seen examples before. It's just a pity that anti-corruptuion zeal was so strong in the Bolshevik Riga post office.