This coin was sold as an error coin, but I am inclined to believe that it is an unofficial issue. The biggest problem is that there are only two stars, instead of three, above the altar. There is also a star in the left field, though none of these from London had this control mark.
Below is the seller's description- note that this coin is actually closer to RIC 222. RIC 263 has the short reverse legend of BEAT versus BEATA.
Constantine I AE3
Obverse: CONSTANTINVS AVG - Helmeted and cuirassed bust of Constantine I.
Reverse: BEATA TRANQVILLITAS - Globe on altar inscribed VOT / IS / XX, two stars above. Star left, PLON in ex. (London mint).
Date: 323-324 AD
Diameter: 19.32 mm.
Weight: 3.12 gr.
References: cf. RIC 263 (Apparently, the engraver misplaced the third star for some reason)
