Unbraked hubs can usually be found on smaller lightweight trailers up to 750kg gross, anything heavier than this requires brakes so a brake drum would be fitted.
Many trailers constructed in the UK and especially older trailers would have the HU4 hub fitted. It fits a standard 1″ diameter axle with the front and rear bearing the same size with a seal fitted on the inner bearing. The PCD is 4″ which was originally derived from the old BMC Mini.
Unbraked trailers originating from Europe use various different bearing and stub axle sizes so its usually easier to identify the hub by the trailer manufacturer. The PCD on these unbraked hubs are usually 100mm or 115mm.
Unfortunately, RTN are no longer manufacturing hubs and there is no alternative replacement at the moment, the bearings are still available though.