After 9 years

Bill,
Are those plugs 1/8″ NPT?

Bill, This sounds like your spring pivots may be the (old) UNEF thread standard that were 18 threads per inch in this size range (dimensions are published on-line). Unusual, but not unheard-of for this era of manufacturing. Measure the thread inside diameter carefully (use a T gauge & micrometer) plus a thread gauge to verify the pitch, then consult a thread chart (table) before assuming the size.   During this period, bolts were made on screw machines and lathes that had fixed gears to set the threads-per-inch, so some unusual combinations came about (such as the 14 thread-per-inch series).

Looking in my 1929 parts catalog, the front mount of the springs is not complex and the threads are likely to hold retainers that give the spring a vertical pivot but prevent horizontal movement.  Check with John Cislak, as he advertised shackle set for the older models and they are likely to be quite similar. Even if you need them custom-made, they are not that complex.

In any event, measure the threads carefully and consult a thread chart (table) before assuming the size.  Taps to clean up the threads in your spring hangers are available, but rather expensive.  Herb