Keep in mind that the spring you choose will determine the centrifugal advance curve that you will end up with. The tension is very critical to getting the proper advance curve. Size and shape is secondary to the measured tension the spring develops when selecting replacement springs.
The early M38 distributors:
IAU4004UT (external noise filter)
IAU4006UT (Internal noise filter)
Use:
IGE24 springs 2 EA
IGE24S 2 spring set.
The M38A1 distributor which was eventually the superseding replacement for both M38 distributors:
Use:
IGB284 springs 2 EA
IGB284S 2 sporing set.
You can check the selected springs you choose by profiling the resultant advance curve they produce in your distributor.
The IAU4004UT and IAU4006UT distributors use advance curve C2-P22.
The IAU4006AUT used advance curve 1046.
Curve C2-P22:
0 deg 275 RPM, 1 deg 385 RPM, 6 deg 925 RPM, 10 deg 1365 RPM, 11 deg 1470 RPM.
Curve 1046:
0 deg 300 RPM, 1 deg 425 RPM, 6 deg 1050 RPM, 10 deg 1575 RPM, 11 deg 1700 RPM.
The reason for the different curves were the L134 for the early curve and the F134 for the later curve.
Surplus City lists the late set but show POR:
http://www.surplusjeep.com/M38A1/m38a1- ... trical.htm