There's some washers on eBay that look interesting. Too expensive of a gamble for me though due to the shipping costs. Obviously not the right match as they're black but still...
They're called ARP special washers. Seems like they're automotive parts
At .625 o/d these are even larger than those I posted. If they're in your back yard though a little trimming would work. But if your modifying why not get a normal flat washer the right size & bevel it? it would be easier & cheaper.
If the orig part is vintage UK (in 1970's) then the part code is going to start BS Sp as far as I've been to get so far. BS stands for British Standard & I think the SP stands for special purpose.