I cannot make a comment regarding the accuracy of (the bobbin website I posted earlier)'s statements. I had run across the website a while back, and liked their method of measuring. My main/longest owned machine is a Singer 403. My mother purchased it (used), to the best of my recollection somewhere around 1960-1962, I was between 4 & 6 yo. In 1974, mom upgraded to a newer (I suspect plastic guts) machine and the 403 became my HS graduation gift.
My 403 takes a 66 class bobbin. I imagine the 66 metal ones were the original ones we used. Later, as plastic 66 became available, we used those as well. I don't really notice a difference. I noticed what they said about a 403 taking a plastic "apollo" bobbin. *shrug*
I bought a box of mixed sewing machine junque from shopgoodwill.com, and there was quite a miscellany of bobbins in it. So, I was looking for a way to quantify *what* I had so I could sell those I don't need here on the QB, for those who are looking for specific bobbins. I did run across a $1 caliper at my local lumber yard, and bought it, so I can start quantifying my misc. bobbins.
I don't normally buy a box of misc stuff like that, but right in the middle of the biggest pic what what I was convinced was a penguin. It wasn't in any of the detail pictures. THAT was the reason I bought that box. However, they removed it from the bundle before shipping. I took careful inventory of what arrived using the pictures, twice. Whatever it was, it was NOT shipped. Dirty Dogs. Plus, at the same time, I bought a second mixed lot which had several spools of serger cone thread, which I will use (I make my clothes). But, they did not respond to my request to combine the two auctions to save me on shipping. The auctions ended within a day of each other, from the same location. Both boxes were half full. It will take something I REALLY want for me to do business with them again.