Seeing the Nymphs Naked?

[Lyceum]

legionary centurions were often cultured chappies who knew their poetry.

There's the line in Martial (11.3) about 'the stiff centurion in the Getic frosts' reading his poetry (sed meus in Geticis ad Martia signa pruinis / a rigido teritur centurione liber), but this may have been another of Martial's little jokes...

most centurions were literate just to the point of fulfilling their duties.

There were all sorts of centurions - many of them directly commissioned from the equestrians and therefore educated men, others who spent their whole careers in peacetime, and so on. So not all of them were the barking hobnailed drill-sergeants of popular imagination!

This has to be one of only a few hand fulls of gravestones belinging to a primus pilus. Do any other centurion gravestones resemble this one?

A quick search on the epigraphic databank brings up 140 inscriptions to primipilares, and a more careful search would probably find more. But I've never seen another quite like this one!

You're right in that the makeup of Centurions was quite varied and that we can in no way assume illiteracy. I mean..not even slightly.

Those lines most likely refer to recitation by the way, not plain reading (as every study on ancient literacy shows; it was a tiered process). In order words, don't take it to having difficulties in reading (which we know the basic school systems did quite well at teaching) but on reading poetry. A wholly different affair dependent on a) practice and b) almost always required participation in the higher echelons of schooling, rhetoric, composition etc and would be limited to the wealthy.