Thanks, gentlemen. Charlie, no code letters on the serial tag other than a "J" suffix, which I think means a Rockford clutch. I think the high gear must be after-market, M&W maybe--I know they made a lot of different things, although I haven't previously heard of a high road gear. And Paul, no worries about being careful--I don't plan to get it up to that speed often if ever, just too easy to bounce myself right off the tractor if I hit a bump or something. This test was one thing, short duration in ideal conditions on a known smooth road, but just cruising at that speed would be something else. Years ago I drove my other Super M to a local tractor show, taking the back roads, and 15 mph or so was more than plenty at times. This new tractor seems to be in like-new condition, steering tight as can be, good brakes, etc, but still, 27 mph doesn't leave much room to recover if things go wrong.
I also wonder about the practicality of such a high gear to begin with, in a non-synchronized transmission. I don't think I could start out pulling much of a load, so I wonder what the high-speed transport capability really served.
In any case, I'm still happy as a clam with this tractor. Maybe in a few years when I find a more rural setting I can use the high-speed gear to drive over there...and then use the lower gears for more satisfying work like mowing, fenceline maintenance, hauling kids around in the wagon, etc. :-)
Dean