f stop on older mamiya 55mm lens

I just bought an older black 55mm mamiya lens from an auction. It

seems to be in very good condition except that the f stops don't

"click" in when you adjust the apeture ring,although the apeture seems

to be working fine,that is if you set it to f11 it will stop down to

f11,you just don't get that positive lock.Is this normal on the older

lens or is something wrong with the lens? Thanks

Some black lenses have full stop detents, some don't. Click stops seem

to have been a later production feature.