Mr. Flash_in_the_Pan,
We have the Lyman Trade Rifle (flint) that we ordered a 28 gauge barrel for, from TOTW; made by Green Mt.. Sat we made the 50 mile trip into the Post Office and picked up same. Was reviewing you post and decided to give it a try this morning. As forewarned, had to remove (file down) a small amount of metal from the sides of the hook so that it would slide into the breach face. Additionally, had to remove a small amount of metal from the end of the ram rod guide (track) where it met the stock for necessary clearance.
Of course there was over a foot of snow covering our bench mounted vise but the temperature is a very comfortable -36 degrees below zero with a mild breeze and it is not snowing. The Sun won't be up for a couple of hours as yet as it is only 0930hrs (AM). Such is life and it promises to be a beautiful day.
There is a small gap between the end of the barrel plug face and the breach face but everything seems very solid. Perhaps if we removed a small amount of metal from the end of the hook, it will set flush. Also the flats are not as wide as the corresponding surface of the hooked breech; to the naked eye.
Realize we are talking apples and oranges (GPR vs LTR) but hope we have not wasted your time with this post.
Best Wishes