The key here is to determine what configuration of gun tank you want. There were almost 2000 Ram Mk IIs and they went throught a pretty hefty number of modifications in the two short years they were in production. The Ram OPs were the very last Rams built and had all of the last production configuration items plus a couple more because they were OPs. There were only a few Rams that matched the configuration of the OPs, under 300, so when you see a Ram gun tank in a photo, it will almost certainly be configured differently than the OPs.
If you have a particular vehicle in mind, let me know the census number (CT #) and I can give you a list of the external changes compared to the OPs.
Or, of course, you can buy my Ram books from Service Publications and have the list yourself...