The drums on the series 1 x 11" (the ones that did have them IIRC some 107 did have 10") are the same however the rear cylinders, shoes and backing plates are different. (They are the same as early SII 109" but not SIIa 109)

The rear have a single bottom adjuster and a floating cylinder so if you have a problem with the rears you may find parts are hard to acquire. It is sometimes easier to retrofit the rear 11" mechanism to the later type.

Diana