I'm pretty sure Slingerland's switch from brass to steel hoops was around 1970.

Both hoops look almost exactly the same, so I don't think sellers of drums with mixed hoops are usually being dishonest...it's just that when you need a replacement hoop, and you find one that looks right, you use it...

I just restored (cleaned up) a '70s set that was missing a 12" hoop...I borrowed a COB hoop from a '60s set...if I forget about it, it may stay like that forever...

AND...Slingerland never used brass for their lugs...they were always "pot" metal, going back to the '30s.

I was curious about this, so I looked it up in Cook's book last night. 1973 is the date he gives. Seems kind of late to me, but what do I know?

I've actually found several 'errors' in these books....Cook's and others. They do their best and put what they know into them. It's not always right on the money, but it's better than nothing.

For what it's worth, I saw the same notation on that page.