I think the confusion stems from your use of the term "Code Zero". A Code Zero sail is a very large sail, similar to an asymmetric
spinnaker, but with a low-stretch
line integrated into the luff. The integral
line in the luff allows the sail to be flown with a taut luff to higher wind angles than a typical asymmetric chute, and also allows it to be deployed and doused with a
furler system. It is normally flown as the foremost sail, forward of the genoa, while close-beam reaching in moderate wind conditions.
Based on the information you provided above, my best guess is that the sail you are describing is either a
spinnaker forestaysail ('staysail) or another sort of 'staysail with an integral stay. The
spinnaker 'staysail is sometimes flown in conjunction with the conventional
spinnaker when reaching (as opposed to running deep). Unlike most conventional 'staysails that are hoisted on the forestay, these sails are set on the fly. With the integral "stay" sewn into the luff, it gets tacked to a deck pad eye and then hoisted from its bag by the 'staysail halyard.
Spinnaker staysails are not too commonly seen these days, but on boats of your era they were not unusual. However, the two-to-one purchase at the head of halyard (which allows the luff/stay to be cranked nice and tight with a conventional mast/halyard
winch) suggests to me that this might be more of an upwind/reaching 'staysail, rather than a
spinnaker 'staysail (which does not usually require a very tight luff).
Hope this helps some.