Maggie & Keith from the superb http://web.ukonline.co.uk/megalithics/index.htm website added this helpful research to the discussion:

This 278 thing is starting to drive us nuts. We did a RCAHMW area search for 700m around SH 722 746. "Moelfe/278" is 72202 74630 and the Druid's is 72292 74658, these are our GPS figures and with the 30min ish averaging times will have a maximum horizontal error of 3m.

The search shows five Stone Circle entries for SH 722 746 entitled Penmaenmawr Stone Circle C - G and one entry for Penmaenmawr Stone Circle (no letter) at SH 7219 7463, the easting for this reference is definitely 100m out!

At first we thought that the letter listings were just different reports dealing with the Druid's Circle, then we noticed further listings up to M with categories such as Cairn and Enclosure.

A Ring Cairn is listed as an enclosure at SH 7216 7457 and we fancy this for your find and what Lynch calls Circle 278.

Moelfre Cairn is listed as SH 7172 7441 which is nowhere near our GPS for "Moelfre/278".

We extended the search out to 1km and there were no entries for stone rows or alignments from RCAHMW, CADW or Gwynedd Arch Trust.

So we hit the library and found the excavation paper for 278, from the description your find is definitely Circle 278. If you're interested the ref is Griffiths W.E., Proc. Prehist. Soc. 36, p305-18
1960.

Although this solves the 278 problem, it makes the identity of "Moelfre/278" an even deeper mystery. Even the Gwynedd Arch Trust entries (which are all 8 figure), show nothing at this spot, of course these august bodies may not have been using GPS, which will put all of their "professional" map fixes into question.