For future reference for anyone else: Measured the old belts in one of the decks. There's one flat belt, medium (4mm, FBM) that measures 38cm, but it's a little stretched so let's reduce that by 10% makes 34.2cm = 13.5 inc. I'm guessing an FBM14 will be the right fit.
There's a square belt, small (SBS, 1mm) that measured ~14.5cm - 10% = 13cm = 5.1inch so that'll be an SBS5.1
Tape counter belt was missing from my box, yet to try something for that.
EDIT: Confirmed: FBM14 is a good fit, as is SBS5.1.