I'm thinking it's books as in "keeping the books" aka accounting. I think (and this is just coming from general archivist horse-sense from reading a lot of old-timey language) it is a reference to their accounting practices, or the lack thereof, facilitating such fantastic savings for you the consumer. Read it as a bit of advertising puffery: Their prices are so low that they can only keep doing them because they don't have any bookkeeping showing they're losing money hand over foot! I've seen variations on this "crraaaazy deals! don't tell the accountant!!" advertising in car sales sometimes today... But it being applied to Hot Discount Meat is pretty novel, can't say I'd be attracted to it, but neat find! :)

I've always heard that this was a firm way of saying the store did not give credit, was a cash only store since they had no way to keep track of tabs.

I don't really have a source for the sign, but found the term here used that way.