This google search:

http://www.google.com/search?rlz=1C1CHMA_enUS361US361&sourceid=chrome&ie=UTF-8&q=leading+overpunched+sign

has a number of pages that hit,

Only http://www.uni.edu/coboldoc/cobrm_042.htm suggests there really is a leading overpunched sign. It suggests that CDO (an Oracle Cobol technology) supports SIGNED NUMERIC LEFT OVERPUNCHED l s, equivalent to Compaq Cobol S9(m)V9(n) LEADING, both of which mean zoned with leading overpunched sign, from what I can infer.

I've never seen such data, but this suggests there are Cobol systems, or were, which can create this format. Whether anyone using such compilers has ever exercised this format is also in question of course.