I hankered after one of these ages ago to fit the gap between my 9 and PW (20 yards....bit too much!)
Never got one (turned down an unused BB for about £30)....and now I have new irons, it wasn't an issue. I found a better/modern set and bought a GW instead.
Since a lot of players didn't use their 3 irons and rescues were still in the early stages, it wasn't such a crazy idea.
When i first started playing in the early 80's my dad bought me a full set of second hand Henry Cotton blade irons 2-10.
The 10 iron was as most others have already said a PW.
Although these irons were a little too much for a beginner, once mastered they were a joy to play with.
Thats alright Tony- quite understandable I know the feeling having worked the last 10 days in a row!
I'm not sure about it being a marketing gimmick- I wonder if it was just Callaway that made 10 irons? They are certainly the only brand I have ever seen one from in the flesh. This could well have been an integral part of the set when created, but as you suggested it could have been a similar ploy to the current tactic from manufacturers that are making the gap between PW and SW huge and forcing the purchase of a GW, where they have just altered the lofts to compensate another club.
THere use to be a lot of company's in the 80's including Slazenger, MacGregor and both Callaway and TM when they broke through who all had iron sets from 3-10. Mostly they were PW's but then as alluded to the lofts changed a little and a PW was added plus a 50 degree 10 (what would now be an A wedge)
Not sure it was ever a marketing ploy as such as I remember my dad having a really old set of irons as a kid in the 70's (can't remember the make) and they had a 10 so it has been around for a while