Dumb question, but I just wanted to double check myself.
It's my understanding that a 126mm rear hub designed for a thread on freewheel will take either a six or seven speed freewheel, completely interchangeable.
I'm building some old Mavic 506 hubs into wheels for a bike intended to be seven speed with friction shifters. I grew a seed of doubt when I saw the 506 hub listed as "six speed" whilst the 501 was listed as "seven speed". The difference in measurements between the two on the freewheel side is 1mm.
Someone reassure me before I lace these up.