The BPM bellows system around in the 70s was a versatile system with mounts to accommodate most camera and lens fittings. But it wasn’t the only one. The likes of Soligor, Hama, Russian, Kenlock and others made similar interchangeable systems, but with their own system of mounts. Im not sure which are interchangeable between bellows makers, but maybe the anatomy of a BPM mount may help you find out if the bellows you have can be used with a BPM mount.
Stepping rings are really useful additions to your filter collection. They are slim rings that have a male thread on one side and a female thread of a different size on the other. They are designed to allow a filter with one diameter be used on a lens with a different diameter.
Stepping rings come in a wide range of sizes
You may, for example, want to use a 55mm thread diameter filter on a lens that has a 52mm thread. A stepping ring is what you need.
Stepping rings are sold in two directions – up and down. The direction is always indicated from the lens. So a step up ring allows a bigger diameter filter to be used on the lens, while a step down ring allows a smaller diameter filter to be used on the lens.
Stepping up is rarely a problem, but stepping down can cause vignetting because the filter may starts to mask the optical path, especially if the stepping increment is steep or if the lens is a wide angle. It’s always safer to step up.
The advantage of using stepping rings is that you are often able to use just one size filter on a range of lenses. especially if you plan carefully.
Say, for example, you have the following three lenses:
A wide angle with a 58mm thread, standard zoom with a 52mm thread and a telephoto lens with a 62mm thread. You could buy a set of 58mm filters to fit straight on the wide angle along with a 52-58mm stepping ring for the standard zoom and a 62-58mm stepping ring for the telephoto.
PhotographyAttic has a range of rings covering threads from 30.5mm right up to 77mm: Lens Stepping Rings for sale