Pulled an old rotted out sill off (having removed the old rotted out wall above it--it was too far gone to be worth trying to save) and found under the sill something that looks like soaking wet cardboard, about the thickness of corrugated board. I'm curious as to whether the use of such a substance was ever standard practice.
Upon further investigation it appears to be just a couple of sheets of Kraft paper--whatever it was I scraped the sodden remains off, pressure washed the area (and got all the spider eggs and cocoons and deceased insects and the like off the areas that will be hidden when the wall goes back up), ran a thread chaser down the bolts, which were in surprisingly good condition, and once it dries I should be good to go on the new sill, this time with purpose-made plastic sill seal and pressure treated sills and CopperCoat (Wolman's half-strength Cuprinol-equivalent, the real stuff being unobtainable locally, at least not without waiting for a special order) on the theoretically unexposed areas that were rotted on the pieces I removed.