"The other inbred Mx+ strain, A2G, originated at Glaxo Laboratories between 1942 and 1950 from an illegitimate mating between strain A and an unknown mouse"
Up until fairly recently, animal research was incredibly disorganized. Researchers keeping mice helter-skelter and every which way. This of course led to some problems with reproduceability.
So it's not entirely surprising that some unintended crosses occurred along the way but it's funny to me, to think of a mouse sneaking into the Glaxo laboratories some fifty odd years ago to consummate an illicit mousey romance.
*Don't try finding this baby on-line, oh no I had to go to an honest to god library to get my hands on this paper. Next I'll be churning butter or some such shit.