Five lug 12" toms are a Gretsch thing and they work just fine there and have since the dawn of time. It is odd to see 5 lugs on a Slingerland though. My Nashville 12 has 6 lugs.
One possibility might be that there were very early production drums and they were sourcing their die-cast hoops from Gretsch which would have necessitated a five lug design.
I don't think it indicates that these are a lower series or anything like that, just a production quirk of some sort. They were trying to replicate the Gretsch design to a degree and the 5 lug small toms are part of that sound. Also Gibson bought Slingerland from Gretsch so there is another connection there that might have played a part.
BTW: the tom mount on the RIMS plate is the same one that my Nashville tom has although mine is bolted right to the drum.
I don't see how you could go wrong at that price. I love mine (12/14/18 "bop" set in Green Sparkle).