PM sent
Looking at the stamping it appears that the worker left the year stamp (ingot) out of the machine on a group of stamps. Have tow other examples of this happening at different periods
With that the J would have referred to the spring provider or stamping plant - in this cars an alternative to the normal one used for San Jose and 74 would indicate (thing 67 was not a leap year) March 15th
Typical stamp pattern for rear leaf springs in 1967 for San Jose is - Engineering number - Day of the year (Julian calendar) - Provider - Year