I think the rims are just regular 700c. Any narrowish tyre for that size will fit. I think some are marked as 28". But thats to do with other sizes of tyres and rims that are in the 700 series. Sheldon Brown has an article about rim sizes. I think its also where 29" mtb wheels come from, wide high profile tyres on 700 series rims. The have a diameter of 29", but the rims 700, thick tyre makes the size up. Old dutch bikes have wide tyres on 700 rims, so they have 28" diameter, and they just left the 28" marking on tyres, tho narrower ones dont have that rolling diameter