I would assume that the original "run" of this limited use wheel was supply enough to last through the mid model run for the 67's and maybe for those used for the 68 models too? I have never seen a schedule of production for Kelsey-Hayes but assume it manufactured wheels based on GM's production needs producing some wheel styles daily and others as assembly plant inventory demanded. The database states that this wheel was first "coded" on March 1 1967, so that batch must have been exhausted by December when your wheels were manufactured. What code was put on the Z-28's produced from December 1966 til March 1 1967?
Not sure when the first ones were manufactured but the early wheels were all large font DF. Believe the date coding they are referring to is the inside of the wheel as I have not seen or heard of any spring/67 date coding at the valve stem hole. The earliest DF wheel with a valve stem date code I have observed is the aforementioned 12/1/67.