You could find up to three different indications of date coding on a DF wheel. Refer to the following image in the following
discussion:
Interior wheel stampings (top photo above)
On the interior of the rim, you'll find "15x6 JK". "15x6" is the diameter and width of the wheel. The "JK" refers to the rim
contour.
Just below the "15x6 JK" are the (upside down) codes "B" and "6". The "B" signifies that the
rim part of the wheel was built during the second production run of the year. This could mean the second month but is not always the case. The "6" means the actual day of the month the rim part was run.
A little farther below the "15x6 JK" is the code "K19". "K" stands for Kelsey-Hayes, the manufacturer of the wheel. The "1" is the manufacturing plant, in this case Romulus, MI. The "9" is the last digit of the year, in this case 1969.
Valve stem exterior stampings (center photo above)
It's not super clear in the photo, but the "K19" discussed earlier is repeated in the center photo, just to the left of the valve stem hole. As before, this decodes as a 1969-built wheel.
Just to the right of the "K19" is the number "9" which represents the month of wheel assembly, in this case the 9th month, or September. The "23" to the right of the valve stem hole represents the actual day of the month of the final wheel assembly. To the right of the "23" is the actual letter code of the wheel, which in this example is "DF", a code used exclusively for all 67-68 Z28 15x6 rally wheels.
Spider section exterior stampings (bottom photo above)
On this particular wheel, the same "K19" is repeated yet again on the "spider" section of the wheel, near the lug nut holes. As before, this decodes as a Kelsey-Hayes wheel built in Romulus, MI in 1969.
1967 DF wheel differences
I have about a dozen DF wheels dated from 1967 to 1969. The description above applies to all of the post-67 wheels. My 67-dated DF wheels all have certain differences, pictured below:
In particular, the "DF" is rotated 180 degrees and is written in a larger size font. Also, there are no exterior date codes whatsoever, with only "KH" stamped on the spider section of the wheel. On the interior rim part of the wheel, just below the "15x6 JK", I found "67" (self evident) and "2" on all of my 1967 wheels.
Real 1967 DF wheels sell for approximately twice the price of later-stamped DF wheels (i.e. $150+ versus $75+).