There has been some interest in Hart traps, I am related to Chauncey Hart and his sons and have collected some of their traps. Chauncey Hart was a blacksmith in Unionville, CT and made traps from about 1845 to 1875. Chauncey had 5 sons, 4 of them worked with him in the blacksmith shop.
Some of the early Hart traps were all hand forged, no cast parts and had square jaws with riveted jaw posts.
Later the square jaws had bolted jaw posts. This trap, marked "4 C. Hart & Son" also has a stamp BBH and a crown. I did some research and found that this is an iron brand of an English steel company, Bradly, Barrows & Hall. At least at that time, Hart was importing steel from England. Interesting, I also have a #4 Newhouse slick pan with the same "BBH crown" stamp on the base.
This trap marked "Chauncy Hart 3" on the base has an unusual spring and cast jaws.
These traps sized 6, 3 and the smallest unmarked, all have an oval lower spring eye.
Here is a trap with cast jaws, almost 6" jaw spread and 22 small cast teeth, 11 on each jaw.
This is the largest Hart trap that I have seen, it is not marked but is identical to a 5 Hart that is marked. The large trap is almost 29" long and 12" post to post.
Here are some very special Hart traps. The 3 on the left have sheet metal springs with a cast upper eye that is bolted on. The 2 traps on the right were patented by two of Chauncey's sons, George and Hubert Hart in 1864. they are a dog-less design.
Latter models started to look like the Newhouse trap to compete in the market. A lot of Hart traps did not come with chains, so you will find them with what ever the trapper put on them, though, some did come with chains attached.
To compete with Oneida Community's Newhouse trap, some traps were advertized as "Oneida Pattern", a few Hart traps were marked on the spring "Oneida Pattern".
This trap has a spring that is cut from sheet metal but has a forged upper eye.
This is the only trap that I have that is stamped on top of the pan.