I know that at one time, Ruger made an 11rd mag. for the P91/P944 series.
I'm guessing that it was made from the introduction of the P91, to pre-Sept. 1994 only?
I seem to remember it's packaging labeled as P-17, and I think that they were in white boxes (for LEO), or grey cardboard backed...
It is probably a early 1970's era Rossi copy of the the Smith 36.
The model number most associated with this one is the model 68
They came in the short barreled, (around 2"), and the 3".
I don't know what years that D.C. based importer brought them in.
I personally think they are a bit better...