Kolchak's Express

Many years ago, Prof. Howard Weinert made me a fine gift of this cover:

Chita to Omsk, September 1919, sent on to Irkutsk. Cash franking of 2.20R on reverse

It's battered, but interesting for a few reasons. It's addressed to Nikandr Mirolyubov, who investigated the murder of the royal family in Ekaterinburg. It's franked in cash (aside: am I the only person on the planet who thinks cash frankings are more interesting than franked letters?). And it has the word "EHKSPRESS" written on the front. After some time I became aware that the cash franking of 2.20R probably included 70k registration and 3x50k postage, so that this would be a triple-weight letter seemed a plausible explanation of the rate. The "Ehkspress"? I assumed it was a plea for this letter to be carried on an Express train, perhaps, nothing more since Express Mail didn't exist in 1919, right?

Flash forward a bunch of years. Masha Chlenova is kind enough to make me lots of copies of Kolchak's postal circulars. For the first time I had primary source material on postal rates in White Siberia! Two sets of rates, as surmised, one starting in late April 1919, the second in October 1919. The rates were for domestic mail only, the rates for mail abroad still have a few riddles left.
But the surprise was with the October 1919 rates. These included rates for Express Mail, at triple the normal postage. Oho!

The April rates did not mention Express mail, but it could have been introduced sometime between the two circulars of which I had copies (April and October) at triple the domestic letter rate. Could it be that ratty letter was actually Russia's oldest Express letter?

And so we come to THIS letter:

Omsk to USA, late October 1919. 6.50R franked in cash on front

It's marked Ehkspress as well! And it's franked 6.50R which would have been the right rate for an Express letter to a domestic destination. However, this one went to the USA, and the reverse had a further surprise:

4R in Arms types, which would have been the correct rate for a registered letter abroad... Is it possible the letter was assessed a kind of schizophrenic rate because the foreign letter rates made no provisions for Express mail? In other words, the Express service only applied to the domestic leg and only paid for that domestic leg. It took a further 4R to get it from Vladivostok to the USA, perhaps?

Two 1919 Russian Express letters...Who could guess?