The original 1100 tank bag was designed for the plastic tank and the part that goes under the seat goes right under and if my memory is correct was stuck by velcro which was stuck to the tank.
The later model 1100 with the steel tank which I have, had a leather tab that was velcroed onto the tank by a strip of velcro that was stuck permantly to the tank just above the seat line.
As the steel tank on the 1100 and 1150 are interchangeable (at least I've done it) you shouldn't have any problems there.
Personally, I think the 1100/1150 GS BMW tank bags aren't too good, but the one for the 1200GS is a far better design, in fact seems to have borrowed ideas from Touratech!
I onsold my original steel tank BMW bag for a Touratech one and have been a happy person since.
Mick.