2, 4, 6, 8....

Popped along to Bradford Industrial Museum today, well it's a good plan on a grey and dreary January day.  

Came across this doo dah, on a printing thingamajig machine.... I tried to see a pattern in the numbers but stamped my foot and scratched my head in frustration.  Not a scooby do how it would work, or what it does, but I do think the answer is 6 and 3/4........ smiles smugly :)))