Click on the link above for the 870 page of the "Remington Society of America".
They state the Remington 870 was introduced in 1950 with a remarkable 15 different versions offered . . . and a TC Trap grade was part of this first-year introduction. In 1950, the retail price of the TC was $159.95.
In 1972, Remington introduced a trap grade called "All American".
In 1978, the "All American" grade was dropped, and they introduced three different trap grades called, TA, TB, and TC.
You are not entirely correct. I bought my "870TB" new in the box for $169.95+tax in 1972. So the "TB" model was around before 1978. I purchased said clay killer from Neumanns Gun Shop in Detroit, Michigan. By the way I have never had to have the piece repaired either, and shot another 24 with it today. I missed 25 'cause I was laughing too hard at one of the other guys on the line.
FYI......I bought my 870TC, Sesquicentennial (150 year Anniversary of Remington Firearms) version in 1966. One of the only guns that I've regretted selling. Gorgeous wood and the best patterning barrel I've ever owned.
This discussion on 870's encouraged me to dig out my old '69 TC. And, I just noticed it had "MOD TRAP" in very small, lightly imprinted lettering on the left side of the barrel near the receiver. Where they all marked in such an almost indistinguisable method? The bore and choke area has not been guaged so I don't know exactly what it measures but it is a remarkable 16 yard target cruncher. Haven't used it much in the last 20 years but I shoot it occasionally....really enjoy the sound of the 'shucking' when I load and unload the shell. Dick
I have a 52'TC and a 53'TB which is the earliest TB I have seen.
They are both nice shooting guns but the TB is the slickest pumping 870 that I have handled.
I have owned over 10 870 traps and at least that many field guns.