Solid bolster breech was what Bill Large called them and I have made some in his shop and mine.
They have the bolster (nipple seat) and tang all milled out of one bar of steel.
I have no interest in making them.The hook breech if done like the top of the line British** ones makes
it easier to get the barrel out of the stock but no other advantages.
Bob Roller
** Fit is so well done a close look must be taken to see the division between
the tang and plug.