Terry, the Zeiss lenses on Hasselblad cameras do not have the more usual thread for filters etc. Instead, they have a bayonet fitting which is generally shortened to 'bay'. Depending on the particular lens, the bay comes in different sizes (older 80mm ones are Bay 50; the later 80mm I just acquired is Bay 60, having a larger diameter).
Also, the bayonet has two parts, external and internal such that a lens hood engages the outer lugs, whereas a filter engages the inner lugs. Simples!