Art, there is so little definitive information on the 3B that I don't think anyone can say for sure what's "correct"! The various hypothoses all seem plausible, given the nature of these cars (an afterthought, built by a subsidiary of Triumph, using up multitudes of spares, etc., etc.). My personal theory is that tooling for the wide-mouth apron was revamped late in the 3A run, and the smaller smooth letters were generally adopted at that time, already being available for the Herald and soon to be used on the TR4. Most likely that was continued on the 3B, but it's also reasonable to assume that spare aprons might also have been produced with no holes at all so as to suit older cars with the wider ribbed lettering.
It would be great to be able to document original very late 3B models as to what, if any lettering they had. As for your car, I'd be tempted to leave it as-is. It's always easier to add the letters later on (if you so desire or can determine for sure that it should have had them) than it is to fill holes you've drilled.
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