They are not a sidelock. The hammer and mainspring are in the frame, while the sear and sear spring are mounted on the side plate.
My Tobin articles were published in Volume Five, Issue 1, and Volume Eight, Issue 1, of
The Double Gun Journal; Volume 39, Number 12, May 1994 of
The Gun Report; and Volume 34, Number 3 of (the Canadian Journal of)
Arms Collecting. Jim Stewart's article on his collection of Canadian Tobins was published in
Canadian Antiques & Vintage May-June 2016, and was more recently updated in Hunting & Fishing Collectibles --
http://hfcollectibles.com/tobin-shotguns/I have an old book titled
Guns, Ammunition & Tackle, by A.W. Money and others, published in 1904. This book was a gift from Alcott Farrar Elwell to his twin brother Stanley Bruce Elwell in Summer 1907. They were the sons of noted sculptor F.E. Elwell --
https://en.wikipedia.org/wiki/Francis_Edwin_Elwell Pasted in the book is a table of S. Bruce Elwells shotguns. He liked his Remington Model 1900 12-gauge, thought his Parker Bros DHE 12-gauge was a handsome gun but "too complicated", and really liked his Lefever FE-Grade 28-inch barrel 16-gauge "Wonderful little gun, fine simple action - good hang - strong, hard hitter." He also had a Tobin No. 40 Ejector 16-gauge with 28-inch barrels which weighed the same 6 pounds 6 ounces and had the same stock dimensions as the Lefever. Of the Tobin he wrote in the remarks section -- "very poorly made - cheap in every way. Tobin Co., no good!"