I received a note from Jack this morning. Looks like the mystery is cleared up>
Pete (Jack's message follows):
Jill and I steel taped the "300m" this evening, starting from the western end of the course. This nail is at the edge of a driveway where a new house is going up (an older house was razed for this). It is the "adjusted" end of the course, as the other is even with a utility pole, and the original measurements were based off of that.
When we got to the eastern end, we discovered the course was 1.25m over 300. I couldn't believe I could make such a gross mistake in laying out a cal course - after all, my steel tape, nominally 50m, has only about 40cm of markings beyond the 50m point, so to measure an extra 1.25 I would have had to measure an additional 20cm each of the 6 tape lengths.
Then it occurred to me that maybe the nail at the western end wasn't mine, marking the end of the cal course, but another nail put in by some surveyors as part of the new construction. When we measured back to that end, we then measured back 1.25m, expecting to find my original nail. It should be noted the nail there was not a PK or MagNail - no machining on the head, but I attributed that to wear from the construction vehicles passing over it.
Anyway, we could not locate another nail in the appropriate spot, but then Jill noticed a divot, with a smaller gouge, in the asphalt right where the nail should have been. It looked exactly like someone had used a hammer or crowbar to dig under the head of my nail and pull it out (it's possible it was a snowlplow, but that seems unlikely, as it was less than a foot off the shoulder of the road).
I'll go back tomorrow and take some photos. And if the workers are at the house, ask them if anyone screwed around with nails.
At least one mystery is cleared up.
Best,
Jack