COB to COS rims, somewhere there as far as I am aware.

As with most things: I don't know how well we can pin it down. Manufacturers don't destroy all their stock and materials on the 31st of December so they can make it easy for us to date things to the "year" when they started. Changes are phased in slowly over time at the factory and then phase in even more slowly as stock is distributed to shops, and eventually sold to customers. Plus Ludwig believed (to some extent) that they were substituting "like for like" so it wasn't a big deal to them.

They were well within the "specification subject to change without notice" which manufacturers state as an out for changing components. And now we go crazy over the most minute detail.

Disclaimer: there are others here who will have a more accurate idea about this. Whether the idea stands up to scientific scrutiny (you know, lots of well documented evidence) is an exercise I'll leave to those who have handled lots more vintage Ludwig drums than have ever been in my country.

What's a "vertical line" on a FT leg?

Are you talking about the surface modification on the top portion of FT legs which is used to help the mount grip them better? Linear vs cross hatched perhaps?

I can say yes they still use the vertical linear lines. I bought some for a microkit a few years ago and they were linear. Alas, I don't have enough well documented examples of earlier ones to say anything other than early on they were knurled (earlier in 1960s) and by 1969 they were linear. But others will have much more material to work from.

Seems like the classic production simplification...save a little time machining each leg. Ringo effect to keep up with skyrocketing production requirements?

Me too. But I don't have any examples from the intervening years in my meagre collection.

The other thing is that the new ones have a much longer knurl area on them. I haven't got my new ones to measure any more (sold that kit) but from memory they are twice as long. This improvement means you have more adjustability, and that we collectors can immediately spot replacement ones. Two for the price of one. :D