This coin just sold on eBay (not to me). Kind of sad since it only realized $25 and the seller seemed well aware it was something a bit unusual, giving the nominal attribution of RIC VI London 13, but also noting it as one of the earlier London mint products with a date of c.298-300, and giving a multi-owner provenance going back 40 years.
The coin's stats were given as 29mm 10.88g for anyone that cares.
What's got me stumped on this one is that the style appears to be that of Bastien's intermediate group (which RIC omits), which seems to have immediately followed the initial "LON" group, but the legend starts with plain "IMP" rather than the "IMP C" of the "LON" coins and all the intermediate group ones I've previously seen.
I'm curious how Cloke-Toone treats this, and if you'd agree it's intermediate group ? I know Lee is on twitter, but he doesn't appear to be active on any of the boards at the moment.