No, this did not happen only with the
mint of
London. Some coins from
Lugdunum (
Lyon) also do not have
mint marks.
Although
RIC lists this coin with other coins minted in
London, a careful reading of the introduction to the
mint of
London (
vol. VI, p. 113-122) shows the editors of
RIC had serious reservations about this
attribution.
The unmarked folles -- ie without a
mint mark in the
exergue -- can be divided into three groups. After many years of careful study, group I has been attributed to
Lugdunum (
Lyon,
France), and groups II and III to
Britain.
Of group II,
RIC says (p. 115), " It is possible that the unmarked II coins were produced in
Britain either from a travelling
mint, or even from the "C" (
Camulodunum?)
mint of
Carausius and
Allectus, with which there are perhaps some stylistic affinities: the period of issue would fall from c. 298 onwards, perhaps until c. 300 or later."
Of group III,
RIC says (p. 115), " The unmarked III coins are in everyway more sophisticated in
style, and it may well be that they were produced at
London, though lack of signature would be difficult to account for: probably it is best to class them as a British series which, for reasons unknown to us, was struck elsewhere. Their date is between 300 and 305."