The placement of the radiate billon "Q" coins of Allectus within the chronology of the successive issues is frequently assumed to be towards the end of the reign, perhaps going back to the 1890s and the publication of the Amiens hoard.
The C mint does not have any such progression, using, in the main, S/P//C through the reign. With the scarcer S/P//CL it is not known if this is a chronological development or a concurrent/parallel emission.
There is no reason, based on mark development, to place the Q coins at the end of the reign and Burnett (1984) and Besly (2006) both recognise this. What if the S/A//MSL coins are less frequent because they are from the end of the reign and were being produced when Allectus was finally defeated? The Q coins could be a manifestation of a multi-denomination billon radiate coinage from any time during his reign.
Metallurgy
Analyses undertaken on specimens of both aureliani and the Q series in the Rogiet hoard provide an insight into the coinage. There is no doubt that the silver content of the Q marked coins is approximately 50% of that of the “non Q” radiate coinage or aureliani, by weight, approximating to 1.9%, compared to 2.8 % in the aureliani. There is also a difference between the London and C mints of the minor elements with London having a greater proportion of lead than tin, whereas this pattern is reversed at C.
Hoard evidence
Hoards of significant size from this period are rather scant but some tantalising glimpses hinting at the chronology are available.
The 2004 Gilmorton hoard terminates with two coins of Allectus, an S/P//ML Aurelianus and a QL galley piece. Rogiet has so few aureliani, 3 in fact, compared to more than 750 galley coins. The London Aurelianus is again S/P//ML, the earliest mark, the other two are S/P//C.
Of hoards that terminate with the London mark S/A//MSL there’s Burton Latimer, 6 cons of that mark out of a total of 48 for Allectus with no Q galley coins. The Colchester hoard contained 167 Allectus with 22 S/A//MSL and only 3 galley pieces, 1 QL and 2 QC.
Based on very scant data you could suggest that the Q coins were not manufactured in the latter part of the reign, given the association with early marks or that there was some selection in the hoarding behaviour. The evidence is very flimsy though.
The Elveden II hoard that terminated with the coins of Allectus only contained a single Q coin. There was, however, a curious larger module coin, a potential hybrid, that was clearly struck with a smaller module obverse die from the Q series. The reverse of this coin, type PAX AVG, had the field marks S P but, unfortunately, the poor strike means the exergue is off the flan. The bust type was determined by Besly to be from the London mint, using the criteria set out in the Rogiet hoard report, as type B’. Upon closer examination there is the possibility of a C in the exergue. Stylistic criteria applied to the reverse die, in particular the unbarred A’s and backwards leaning figure of Pax may support the presence of the letter C in the exergue.
Overall it looks like there are grounds to doubt the late placement of the galley Q coins in the reign of Allectus, but the picture is far from clear.
[Full references to all the works cited in this post can be found on the bibliography page]