"ought" is a variant of "aught". "aught" is a variant of "naught". "naught" is also "0" or "zero".

"ought" then gets substituted for "zero" like in:

1906 (19 ought 6)

Double 00 buckshot or double ought buck

For wire gauges (sizes) there is single ought (0), double ought (00), triple ought (000), quadruple ought (0000), etc.

All this according to Google searching.

Jim