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There you can see that the registration fee was 8c Jan 1 1893 to Oct 31 1909.
Thank you for that update, although the answer wasn't the one I was hoping to hear. So that puts the total rate for a standard weight international registered letter at 13 cents.
The cover in question opened considerably above the pre-auction estimate of $200, and I and another bidder battled it out, and I ultimately paid an obscene amount for it.
I hadn't confirmed the rate prior to bidding on the cover, or likely would not have bid as high as I did (the major lesson to be learned here)... although I love the aesthetics enough that had I not won it, it likely would be bothering me deeply, since the likelihood of ever seeing a comparable cover is near zero. Time will tell whether my enjoyment of the cover will outweigh wincing over how much I spent.
The revenues used on the following cover were apparently completely superfluous, presumably sent to a colleague/friend who collected stamps, and NOT actually used as postage. 13 cents regular postage + 3 cents in revenue stamps.
But it's not just the revenues used on an international cover that drew me to it. I have several international covers from the 1898 tax period with revenue stamps affixed, and have seen examples of dozens of others. They are almost universally to countries in Europe. Remote destinations are far more scarce. How many examples of revenues on covers to the Straits Settlements will you likely ever see?
Also, the shift of the overprint on the 1c Franklin is neat to have.

But then flip over the cover, and not only do you have a nice Penang receiving stamp, but also multiple seals and a large handstamp from the Anheuser-Busch Brewing Association (the name that the Anheuser-Busch brewery went by at that time), so the cover also classifies as breweriana.

Gorgeous one-of-a-kind cover... but ultimately a bit too expensive for what it actually is.