I have the impression that the #80 bit is the smallest commonly available. Years and years ago I did find a set of smaller bits at a jewelry supply company. They had fat shanks; the drilling part was only about 1/8" long. For model building, frankly, they were virtually useless: they were made of high-carbon steel, and snapped under the slightest side pressure. They also, simply because they were so small, got dull quickly - and were, if I remember correctly, quite expensive.
I'm sure modern industry produces bits smaller than #80, but I question how practical they'd be for model building - in terms of handling, breakability, and money.
I wonder if, in fact, the unmarked set mfsob bought actually goes all the way down to #80. A #80 bit is mighty small. In 49 years of model building I can't think of an occasion when I've really needed anything smaller. A #80 bit certainly would be small enough for 1/700 portholes; in fact it might be a little too small for that purpose. A hole drilled in plastic with a #80 bit can, if you're careless, be filled up accidentally with paint.
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