That is a Stevens Walden PM21 21-piece 1/4" and 3/8" drive SAE socket set and they are very rarely complete:


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this one is over at the post office. I haven't had a chance to go over and pick it up yet. this one is 100% complete:


they did not come with a 3/8" breaker (model 4032). usually the 5/16" hex key is missing from the set.
the "extension" is the 3-inch male-to-male piece and uses the "connector" piece - kind of an odd set-up for such a late-production socket set. it appears that the only thing you're missing is the 5/16" hex key - it's just a standard, regular old 5/16" hex key - nothing special. not a clue why it was included in that set.
I think the illustration of wifey sitting on the back of the Cal Trimmer wearing her sun bonnet while hubby works on the machine is a classic - takes "artistic license" to an entirely new level, doesn't it?
I have not quite figured out what the deal is the the 1/4" spinners- they came in red, yellow, blue, and green. No idea if that has anything to do with "vintage".
(* oops! got a photo image of a PM32 in there by mistake. disregard that one. mine is the second set. I have another one out in the garage somewhere, also complete. BK )