Understanding sizing for old helmets

I understand that different manufacturers helmets will fit slightly different, but....

A lot of the old helmets I see listed use "7 1/4 - 7 3/8" or "6 1/2", etc. What do these sizes translate to? For example, if I measure my head and gets "X" inches, how do I find out if that's a size 6, 7, 8, etc.?

There are conversion charts out there - most helmet manufacturer's websites have them as do online vendors.

This is from the Shoei website:

SIZE HAT SIZE CM INCHES
XXS 6 3/8 - 6 1/2 51 - 52 20 1/8 - 20 1/2
XS 6 5/8 - 6 3/4 53 - 54 20 7/8 - 21 1/4
S 6 7/8 - 7 55 - 56 21 5/8 - 22
M 7 1/8 - 7 1/4 57 - 58 22 1/2 - 22 7/8
L 7 3/8 - 7 1/2 59 - 60 23 1/4 - 23 5/8
XL 7 5/8 - 7 3/4 61 - 62 24 - 24 3/8
XXL 7 7/8 - 8 63 - 64 24 3/4 - 25 1/8
XXXL 68 1/8 - 8 1/4 65 - 66 25 5/8 - 26