It could be someone in the shop built it as a project. I believe they did things like that.
I seem to recall hans mentioning some allowances being made for that kind of thing, and it would be a very plausible explanation for the lack of s/n on the headstock, but I don't think any knowledgable appraiser would recognize it as a
GENUINE factory initiated product without a s/n or some other kind of provenance explaining why there is no s/n.
In fact calling it a "Custom" verges on implying it could be a Custom Shop creation, and those DEFINITELY had some extra identification as to their source.
Even in the collector car market, "custom" denotes something that was NOT built in the factory.