Hi John, looks to be an 'H' the Lincoln one to me mate......I have seen BSA's however a little
later in period (1919 and on) with the Lincoln stamp, but obviously he was a retailer too and so stamped the woodwork with his address / trademark.
The wood to metal joint and pin location are the same from 1905-39.....all stocks interchange....BUT...
It depends on the type of trigger guard, ie. the tang in-letting on the pistol grip area, very early guns had a full length tang, but the majority seen pre 1914 are cast units, with the stock having no inletting.....so the only later guns that these early stocks fit (without modification) are the later standards with the simple tang less guard.
Look at the gun 4th from the left in
this pic, Its a .22 standard from the 20's with this type of sheet trigger guard, but an early straight stock
If you have the steel butt plate as well, you have been given a nice pressie!.......I have seen one of these make the high sixties in price, they will normally make 20-40% more than a similar period BSA one.