Yes, it is interesting. So far, I have not seen or heard of any confirmed Fountain weights by Paul during his Caithness years.
From the seller's information, this might prove to be a first. But I would suggest getting it checked under shortwave uv. If it is Caithness period, it will show as blue under the shortwave (as far as we know, Paul always used the "best glass" [lead-based] for his weights at Caithness).
Nick, if you have not already done so, could you try checking your weight, from all views, against the photo in the Ysart Glass book. Could it be the exact same weight?
I have seen several very similar examples, usually with similar colours and less then 3 inch height and with dome being "less pointed" than many of the other Fountain weights.
I have one of these with much the same red for the ground, probably the same yellow centred + white outer twist, and with a pale purple centred + white outer twist. Size is 2 3/8 inch diameter at the widest point and 2 13/16 inch height. It has no label and no H cane but is scratch signed P. Ysart. It fluoresces the same as my other Harland period weights, signed and unsigned.
I bought mine directly from Paul's last secretary who received it from Paul in 1979.