It is easiest to see on the halfgroats. Halfpennies usually have the mark off flan and pennies are just plain uncommon. The mark is also competing with T, cross patonce and a rare rose reverse, which I haven't seen to date. The first two are also muled both ways and are common in any combination, the rose is paired with the uncertain mark obverse, which is itself never otherwise muled. Given rose was a first issue mark at Canterbury, it being muled with the uncertain mark means that should be at the head of the second issue marks. The frequent muling of the T and cross patonce suggests they ran concurrently and may well be the marks of the Archbishop's (2?) engravers.