Well... here's a completed auction that features a nearly identical short-billed cap with the number "40" on the front. This grouping places the cap with other baseball equipment, so who knows...
http://cgi.ebay.com/INCREDIBLE-LOT-O...item4a9de25a11
I suppose these numbered caps could be either Baseball caps or college beanies. The ebay grouping realized a good price, so I guess those bidders at least felt it was baseball-related.