Dunno about a full tutorial,Steve,but here's a little something...
The first trap grade 870 was called the Grand American.Discontinued in the 60s, it was replaced by the letter grades, which were discontinued before 1980, if memory serves.
The TA had a bit nicer wood than the field models, the TB better yet. The TC had very nice wood, and briefly, slightly overbored barrels.
This led to the practice of a trap shooter buying both a TB and TC, swapping barrels, selling the TC with the standard bore TB barrel substituted for TC prices, and keeping the TB with the overbored TC barrel. My TB is probably one of those.
There were also TD,TE and TF grades, with factory engraving and exhibition grade wood. Some inlay work was done by the Remington Custom Shop.
Skeet 870s were also available, in SA, SB etc, grades. These had mostly 26" barrels and skeet chokes.
Both of these had otherwise standard parts, including triggers. Since most 870s of the time had decent triggers, these rarely required tweaking.
HTH....