I have attached the Section on Mustangs from the Salesman's List Price Book for 1967 Ford Cars. The ones I have attached are from the Book dated May 29, 1967 (Mustang Section dated 3/10/67 and 5/29/67) ; the Options shown in relation to this topic (SSW, tires and 390's) are the same as in the other Book I have that is dated November 7, 1966 (Mustang Section dated 10/17/66).
Several items of note :
1) If you ordered a Mustang with the Optional 390 (S Code) Engine, it came Standard with 7.35x14 BSW tires and 6" wide wheels.
2) If you ordered the Competition Handling Package option, it came with 6.50/6.70x15 Sport Car 200 Nylon Tires with 6" wide wheels.
3) If you ordered the GT Option, it came with F-70x14 Wide Oval WSW Tires, with 6" wide wheels.
4) If you ordered F-70x14 Wide Oval Tires (V8's only), they came with 6" wide wheels.
5) Note that none of the above Options came with Styled Steel Wheels. SSW were a "Stand Alone Option".
All that being said, whomever ordered your car originally, wanted both the Wide Oval Tires and SSW. Thus the SSW Option overrode the 6" standard type wheel that would normally come with the Wide Oval Tires. You would then "sacrifice" the 6" wide wheels, and have to "settle for" the 5-1/2" wide SSW (while not shown in the pricing books, the Ford Master Parts Catalog shows that the SSW were only available in the 5-1/2" width).
But look at it this way, in 1965-66, they were only 5" wide.
As for the difference in your Build Date versus the dates on your SSW, a lot of things could have caused that ; mistake at the Plant ; no inventory when the car went through; damaged (stolen) in transit to the Dealer ; or a more likely and somewhat common scenario, would be that a potential buyer of another Mustang at the Dealership that did not have SSW, and the Customer wanted them as part of the deal, so the Dealer "borrowed" the wheels from your car to make the sale, then replaced them at a later date either from a later built car or by ordering them in to inventory. The latter is a good possibility since your car didn't sell until July. Just some thoughts.
Hope this helps.
Bob