There was a
weight reduction at all of Constantine's mints in 335, at the same time the
Gloria Exercitvs type changed from "2 standards" to "1
standard"
type (presumably because it was easier to fit the 1
standard design on the
reduced size coins).
RIC 242 dates to 330-331 before the size/weight reduction (when
Gloria Ex. 2 standards was being issued).
RIC 275 dates to 335 after the size/weight reduction (when
Gloria Ex. 1
standard1 was being issued).
The only way to distinguish
RIC 242 vs
RIC 275 will be based on size (
weight is unreliable) unless there also happened to be any noticeable
obverse style change (I don't know - I havn't really studied these in detail).
There is a general trend for
mintmarks to evolve from simple to more complex, or in "logical" order, but you can't rely on this. Somtimes issues can be clearly dated based on who's on the
obverse, titles, etc, but othertimes all we can do is extrapolate between known dates - e.g. if there are five issues in a five year period without any known date landmarks, then we might guess they were spread out one
per year. If one issue is more common than another then we might guess it was a longer lasting issue than a rarer one. There doesn't seem to have been any general pattern for how long each issue mark lasted - in some cases they may have been very brief - perhaps only a week or so, but in other cases the same mark was used over a period of years.
Ben