Heathkit catalog question

[Brian Wood]

Does anyone know how Heathkit numbered their catalogs, and exactly when they
switched from the nice, glossy, firm covers to the paper ones?

For example, I have catalogs with the following designations:

1965 -- 810/50
1966 -- 810/60A
1967 -- 810/67R
1968 -- 810/68
1969 -- 810/69
1970 -- 810/70R

Brian Wood, W0DZ

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