mar / Varese, redmadder,
Ancient or original Class 66 bobbins are mostly found in old machines, boxes of attachments and accessories. That is where we've found them the most.
My 201 is mid 1951 vintage, I have 5 66s, a 99K, a Sewmor 404 and a bunch more machines that take the Class 66 bobbins, and so far none has shown any pickiness for bobbins. I have metal bobbins from the earliest solid lathe turned versions to the modern cheap stamped metal ones and the plastic ones. So far, so good.
My bobbins have come from Sew-Classic, JoAnns, Good Will auctions, other machines, and many from my wifes own scattered stash. (Her original Singer 538 uses Class 66 bobbins.)
So, I'm thinking this situation is caused by tolerance stack ups. The bobbin carrier in the machine is on the tight side, and the bobbins are on the large side, so it's a no fit situation.
The only solution to this is to buy some bobbins from a different source. And when you find some that fit, buy a bunch. I suspect that we have a couple hundred Class 66 bobbins scattered between the machines we use and the supply case we keep the extras in.
I won't run out ....
Then of course there's the Class 15s, the 221s, the 319Ks, the Kenmores, the long bobbins ....... Eeeee Gads I'm bobbin poor!