Latvia 1992 Mixed Cover

Another one from 1992. The original envelope is a Soviet era 5kap one. Interestingly without an overprint like the stamps used during the same period. It has been uprated by the addition of 45kap which I assume is the European rate. And then an additional 450kap because it was being sent to the USA. How would you describe this? And just out of curiosity is the added postage done with some sort of hand stamp or run thru a meter of some kind?

In 1992,from March 1 to August 9,the rates for letters under 20g were :
100k for inland mail
300k for surface mail to any foreign country (even Estonia and Lithuania)
500k for airmail to any foreign country

Here is one from November 24, 1992. Franking is 3.00 rublis, including two 1.00 r. overprints. This is domestic mail from Sigulda to Riga.

And that franking is correct.
From August 10 (1992) to February 14 (1993) the rates for a letter under 20g were,in latvian roubles :
For inland mail 3
And to any foreign country 10 for surface mail and 15 for airmail

It looks like 50kap was prepared in advance in both cases. Is the purple 45 and 450 done with a hand stamping device or some sort of meter machine?

(from the specialized catalogue of Latvia 1990/2000)

vayolene awesome, thank you!

For the 5 kopeck Soviet stationery with the 45 kopeck Latvian rubber stamp added the terminology would be "provisional Latvian stationery". Adding the 450 kopeck stamp the terminology would be "uprated provisional Latvian stationery", since the 450 kopeck stamp was added to the stationery at a later point in time (perhaps?).

The 45+5 kopecks were intended for the domestic 1st weight class rate (<=20g).
The 450+45+5 kopecks were intended for the international airmail letter 1st weight class rate (<=20g).

It's really cool with the marks of handling on the backside of the cover you've shown. What remains to be seen is whether any other covers exist of this 450+45+5 kopecks uprated provisional stationery - that has not been sent by Janis Cigelskis.