I may have to replace my vintage Shimano 600 6 speed 13-25 threaded freewheel. Comes from a vintage 1983 Bianchi road bike with shimano golden arrow components.

I took the old one apart, cleaned and re-greased the 90 odd bearings. Seems to work fine but won't know till I ride it. So if I have to replace it what do i get? See picture below.