Hi Charlie,
I've been a regular user of your 17 x1.2mm belt, but I've not always been successful with it, even in the same machine/drive that has previously worked. The Sony part that I'm replacing is 0.8mm square cross section. Is 17 x 0.8mm a size you might be able to source?
The Sony part is 4-227-025-01 (not that this is much use to anyone, but recording here as the part for MDM7 series drives used in many Sony consumer and pro machines from ~2000 of Type R or Type S using 2662 DSP).