Optio spes/ad spem ordinis

[Carus Andiae]

This is probably a really stupid question, but...

I'm familiar with the term 'optio ad spem ordinis' ("an optio marked for promotion to centurio and awaiting a vacancy", as defined in MC Bishop's Ut Milites Dicuntur).  However, I've also occasionally seen the term 'optio spes' used (seemingly) to refer to the same thing.  Could anyone explain what - if anything - is the difference?

[Frank]

I just know the other term Optio speis. Genitiv of spe = hope.

I always thought it is a synonyme or abbreviation. At least there is just 1 rank of an optio awaiting a vacancy. Such an optio had perhaps an higher image, but no different military role than an ordinary optio. Payment might differ, but rather depending on where this optio ad spem ordinis comes from.

PS: Well, it is not really a rank.

[D B Campbell]

However, I've also occasionally seen the term 'optio spes' used (seemingly) to refer to the same thing.

The correct term is optio spei, which to all intents and purposes means the same as the more long-winded optio ad spem ordinis. You can see it (e.g.) here (on lines 5-6).

[Carus Andiae]

Thank you both. Somewhat clearer, now.