Jackie:
Usually the only differences between a trap grade 870 and a field grade is the wood, and an addition of a middle bead on the barrels.
Field grade 870's usually have a lower stock for hunting purposes, and the wood is less figured. A trap grade 870 the wood was graded as TA, TB, and TC, as well as higher grades, TD, and F grade. The TA had the least figured wood on a trap stock, and TC had hand cut checkering and nicer figured wood. The TA and TB had pressed checkering.
The TC grade with the nicer wood weighs more than the TA and TB so Remington took the extra weight out of the pistol grip to make them weigh roughly the same. The TC pistol grip is a bit smaller than TA and TB.
Years ago, two types of trap stocks were offered, a monte carlo type stock and a non monte carlo stock usually refereed to as a straight trap stock. The comb height is higher compared to a field grade 870. Most trapshooters want a trap grade stock with the higher comb so the gun shoots higher for rising targets on the trap field.
Regarding the barrels on a trap grade 870, they came usually with a 30 inch full choke barrel with a middle and front bead. You could also purchase a trap grade gun with a 30 inch modified choke if you wanted. These barrels could also be purchased in length of 28 inches. Remington use to offer numerous barrel lengths with fixed chokes over the years.
Hope this helps you out a bit.
Steve Balistreri