Another thing I just noticed when looking at OCRE is that RIC 251 comes in two variants - one (presumably the first) with an "X" on the altar, and the other with a garlanded altar. I assume Bastien noted both types? It makes for a nice progression from simple to more elaborate through these early altar types, with the notable switch from CONSECRATIO to MEMORIA FELIX in the middle.
Below: BM, Vienna, BM, eBay specimens of RIC 251 (X), RIC 251 (garlanded), RIC 267 and RIC 268.
It's interesting that only Lyons had such a variety of these types for Divus Constantiius. Why Lyons?!