Hello, Shaun and Andrew,
'Good on you' for offering to make an 'off the shelf' tool which would save the time involved in making up a 'special' for some inconvenient 'bad wrench clearance' applications.
Your idea is a good one, which had been made up by some small tool company, back in the 1940's and '50's, but has been out of production for many years.
I've been retired for quite a few years, now, but I do remember, vividly, some of the special tools we had to make up for 'tight clearance' applications. I remember stamping 'Heath Robinson Patent' on a few of them, as a comment on their appearance, but they got a job done, which was all that mattered.
If I may, I'd like to offer a couple of thoughts.........Firstly, I'd tend to suspect that you are wasting your time and dollars with a patent application. The clearance cut on the hex may......just may.....be 'novel'....and, theoretically, you just might just 'squeak by' on 'novel combination', but there is a lot of 'prior art' in hex shank tooling.
If a large firm chooses to infringe your patent, its your responsibility to sue for infringement, and they will keep you in litigation for some great length of time, stall, stall stall, with various continuances or whatever other legal trickery, while the sell a cheap 'knock-off' of your design. (ask me how i know this.....its no joke)
My guess would be that simply offering the tool, as a best quality specialty item, in sites like this one, on the internet, will get you all the sales you can handle readily. Its quite possible that some tool-truck sellers will order a few to offer along with their regular lines, to be sure.
This one is an excellent specialty tool, actually, but it will have a relatively small market, amongst the relatively few mechanics who will appreciate what it will do for them.
I would be reasonably confident that the old Apex patents on hex shank tooling expired years ago, for whatever that's worth.
Secondly, any stainless is needlessly costly 'over-kill' for this part.
6150, with suitable heat-treat, is optimal, but, for this purpose, 4140, again with a suitable heat-treat, would be more than 'adequate'.
Put in a query on the 'practicalmachinist' internet board, and someone will refer you to a forging shop which can make up the blanks for you, at a realistic cost level. That part is about as easy a die-sinking job as may be imagined, so tooling cost should be low.
Remember that there are many wrench users who are accustomed to 'inch' fastener and wrench sizes, so humour them. It won't cost that much more to offer the tool in a choice of inch or metric dimensions.
Best of luck to you, you've a good design there.
cheers
Carla