This directory summary contains a couple of errors:
"Large Saucer-shaped Blip" should read "Large Sausage-shaped Blip",
and the conjecture here
"...a large, sausage-shaped blip [arc shape due to radar display?]"
cannot be true because the major axis of the blip was always perpendicular to the direction of displacement, which was initially along a constant-radius curve heading roughly west around the north of the radar site, then along a radius directly away from the radar site. Considered alone, the latter might be interpreted as the usual beam-width target arc, but not blips that were initially elongated radially.
I suspect some kind of radar interference in this case because of the constant relationships of the echo paths first to the scope centre and then to the scope radius. Similar radars with very nearly identical pulse rate and scan rate might cause something like this as the very slightly asynchronous periods move in and out of phase.
It's hard to work out exactly, but other clues in favour are a) the report that permanent ground clutter echoes strengthened noticeably when the blips were on the scope and b) the fact that radio interference swamped the communications net at the same time, and c) the fact that other nearby radars (including heightfinder radar at the same site) did not detect any targets in the area where these blips were detected on the CPs-1.
These points together suggest the possible onset of super-refractive propagation conditions, so that the radar horizon and the VHF radio horizon were abnormally expanded, allowing signals or hash from distant transmitters to spill in. The nearby radars with coverage of the same area would, because of their proximity, be deliberately de-tuned from one another by a few tens of MHz otherwise interference would be constant problem. Thus only one of the radars in the area happened to be working the right bandwidth to be affected.
Not proof, but a reasonable suspicion?