I recently picked up this coin from Aquileia. It is always called a cross on the reverse, but most of the time they look rounded at the top (note how the top of the example below even resembles the top of the banners), almost like an ankh, but maybe a crux ansata which was a Coptic adaption of an ankh. The second picture below is the only example that I have had out of about 6, that looks like what I think a cross should look like. Anyway, I know most like to believe this is a Christian cross, but I like to keep an open mind. Mabe it is something else, which doesn't preclude it from being a religious symbol. Strangely, the next control mark Aquileia used was an F; which I just picked up an example of that type also (third coin below). Well, I am just thinking out loud.
Constantine I
A.D. 334-335
CONSTANTI-NVS MAX AVG; rosette-diademed, draped, cuirassed
GLOR-IA EXERC-ITVS; Two soldiers helmeted, stg. facing one another, reversed spear in outer hands, inner hands on shields resting on the ground; between them two standards, cross between.
In ex. AQP
RIC VII Aquileia 124
Constantine I
A.D. 335-6
CONSTANTI-NVS MAX AVG; rosette-diademed, draped and cuirassed bust right.
GLOR-IA EXERC-ITVS; Two soldiers helmeted, stg. facing one another, reversed spear in outer hands, inner hands on shields resting on the ground; between them two standards with F between.
in ex. AQS
RIC VII Aquileia 131