I'm surprised you say this. We're probably looking at exactly the same coins so I suspect the difference is in definitions. I'm talking about five full steps, not six. I'm not sure any of those I've seen would go six steps. There is a lot of trouble with marking and weak spots knocking coins out of contention. About two out of three SMS nickels have enough marking to be excluded. Unlike most modern nickels there's a little correlation between being free of marking and having a good strike or they'd be much scarcer. With many dates it seems the best made nickels are put aside for extra handling. The sets I cut up recently were far from random. They were sets that were cherry picked for many years going back to 1975 and FS was one of the criteria that was used to select sets. Indeed, I periodically went through and culled out substandard coins and sets. If I greatly exaggerated their incidence it was not intentional. I also used to chase down Gems by zip code since the mint made them in small batches and they went to the same zip code. I did a lot of driving and looking through dealers' mint sets but it's hard to believe that the universe of sets I've picked is vastly different than what you've seen simply because most of the rejects I picked over would have been destroyed after i saw them. When mint sets used to hit the market they would accumulate in massive piles of inventory until the dealer cut them. Even today few mint sets are sold by coin shops. The reason few coin shops have much inventory is the sets have mostly been destroyed. I found the '66 FS to be virtually "common" with a nearly 1% incidence. The '65 is tough and the '67 very tough. IMS the '67's weren't as strongly struck as well and it's possible that some wouldn't even consider them FS. These are all in my safety deposit boxes so it might be a while before they see the light of day again. All my coins are for sale but not all at todays prices. I've sold many Gems but mostly the finest and those two grades off the top. One grade from the top is greatly undervalued today so I've sold very few. I'd be most interested in description of the SMS nickels you're looking at.
Here's a close one in a 2x2, a bit weak under the 3rd pillar but it is there seen in hand but weak. Taken with an Android phone just now. There's a 1965 SMS pic with nice steps in collection. Leo Jefferson Nickel Collection for sale. — Collectors Universe