Ray,
Don't be including Partical Separators in your modex theories now. Those are apples and oranges. If it didn't come off the line with a separator, then it was just an AFC (airframes change) to be applied at the next overhaul. If it was important enough a NARF team would visit a squadron and apply the AFC in the "field". I don't think it happened here. I flew aircraft without separators right up to the end. As they were nearing the end of service life it just wasn't deemed that important. There might not have been any "Kits" available either.
Anyway, separators have nothing to do with MODEXs. It only shows that the AFC has been applied.
By looking at the updated spreadsheet one finds a "pattern" to MODEXs and BNs. Since we don't have the policy letter available to us, all we can do is draw conclusions.
I conclude that HT-8 most likely gave the low numbers to the TH-57s and the TH-1Ls started with 35-78. When they transfered to HT-18 in 1972, they had the 100s block and went from 136-179 for the TH-1Ls and UH-1Es, UH-1Ds used the 100-135. That is my theory right now.
Remember that MODEXs have absolutely nothing to do with BNs. It's easier to FIND and aircraft out on the line with large MODEX numbers than it is to look at the small tail number. Note the Armys large local ID numbers on thier Rucker training aircraft.
You busy moving?
Regards