After your initial post, Marc, I also went and looked at the EMP database to cross reference and look for any additional details.
What I also found is that a number of stills similar to yours were sold (blank borders with no identifying info on either the front or back) but were described as 1951RR stills put out by Realart. I wonder how that fact was also determined or learned, as in theory, yours could also be Realart RR images, too. Some have writing or some address labels on the back, but nothing that reads Realart specifically. Or maybe there is some small Realart identifier on these stills that I'm not seeing?
These are the 3 that caught my eye:
http://www.emovieposter.com/agallery/archiveitem/6891536.html
http://www.emovieposter.com/agallery/archiveitem/7017998.html
http://www.emovieposter.com/agallery/archiveitem/15651314.html
I don't know that it would be that easy to attribute these to Realart, in part for some of these reasons:
1) many stills have no identification on them what-so-ever and frequently blank because stills are printed for different purposes.
stills exhibited in a theatre will generally have the title & date etc & an nss slug.
stills printed purely as promotional items in presskits may not
stills sent to news outlets generally have no imprint as they don't come from NSS, they come from the studio. These stills most commonly have typed or printed snipes on back
key sets bear no identification quite often other than a copyright date & company slug
these rules are not always the same, as different movie companies may not use the same methods and even different NSS distributors may not use the same methods
2) unless they are in an original Realart envelope, with other stills that have Realart identification, that would be correct that they cannot be identified definitively as Realart stills and you may not be able to tell (for instance) if they were instead Shock Theatre promo stills (TV) of the late 50s or stills printed for some other use such as magazine articles and the news service requested pics, though generally, news services stamped the back of their file copies
and there are many other reasons for stills to be printed.
3) the # on the stills is not a useful indicator of release, as the 310- is used on all releases to my knowledge.
It would be impossible in my opinion to definitively date these stills
4) as a result of this it would have be best to describe them as '1950s' or even 'unknown vintage'
keep in mind, as I noted, these rules vary from company to company, NSS to NSS, purpose to purpose, decade to decade
but there is no definitive way to say these are Realart stills, promotional stills, or otherwise.