High value Tridents are normally only seen on Money Transfer Forms and, less often, Parcel Cards. Often they are clipped or punched.
Very rarely, they can be found on letters like this Cash on Delivery letter from ODESSA 5 11 18 to ROVNO VOL 8 11 18 with typical blue arrival cancel. Remarkably, the franking appears to be correct using Alexander Epstein's Tariff table published in Ukrainian Philatelist 2004:
3 kopecks for each ruble or part ruble : 342 x 0.03 = 10 r 26 k
Inland letter , first weight step 25 k
Registration fee 25 k
Total 10 r 76 k
The franking is provided by no less than 5 Odesa Trident types:
Type 2 on the three 2 kopeck perforated stamps
Type 3 on the 50 kopeck perforated
Type 4 on the two 10 kopeck perforated
Type 5b on the two 3r 50 black and grey on vertically laid paper
Type 6b on the three 1 rouble perforated
I believe that there was once an archive of letters from this period and to this address in Rovno; I am sure I have seen other examples. Some of them may have had the front removed, so that they are only cover backs. At one point, I had in my collection (ex Lindenmeyer if I recall) part of a cover back to Rovno with a high value franking including a couple of Kyiv Type E stamps. One or more stamps had been clipped from the cover back. I let Ron Zelonka have this item: whatever Kyiv E was, the existence of the cover back implied that it was a non-philatelic type.
Postscript 17.01.2012: This item has now been SOLD