What did cable installer use to tap onto my service ground?
Thanks to the "split" term in ThreePhaseEel's answer, I finally chanced upon a close match via an image search. Apparently these are called "vise connectors".

For example, GMP Tools calls theirs a "Ground Vise Connector" (part number 77966) and MacLean Power Systems sells a "Bronze Vise Connector" under a variety of variants (e.g. catalog numbers BVC-2/4/6 depending on size). Allied Bolt calls them a "Bronze Vise Compression Connector" (e.g. part number 862), and also offers something very similar they call an "H Connector".

I'm not sure how they compare to the split bolt. I suspect they're a little handier for cable techs since the halves don't need to be fully separated to install, and/or are better for bonding two different gages of wire. In my case, it looks like the local home improvement stores tend to carry the split bolts and not these vise connectors.