Spanish as with Italian 20's tend to be bored at .626" . I dont have my conversion chart to hand but I bored a set of Spanish barrels on Friday and that was the equivolent marked size.
Most 12's come in between .720"/18.3mm and 728"/18.5mm , 18.4mm/.724" is the norm but as with any production there is a variance .

Does it matter ? Chokes should be based on bore size and a couple of thou either way will mke no diferance .