Tubular tires and tubeless tires are different.
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You don't need tubeless tape, you need different rims. No tubeless tape ever made will help you. Tubular rims don't have any of the features that would retain a clincher tire. This is precisely why you couldn't measure an inner width. The only thing you can mount to those rims is tubular tires, which are very much a dying breed.

Now, tubular glue tapes are a thing, which I guess you could Google if you want to mount tubulars. I strongly recommend you do not do this unless you know exactly what you are getting into. The skills required to mount tubulars are learnable, but they're also dying out so there's not many people to learn from.