HP 35s checksum repeatability observed

I have had the opportunity to work with a few 35s models in recent months.
The well documented checksum problem with the 35s is a great disappointment, but I have seen some repeatability when programming multiple 35s calculators.
I have 2 calculators (SNs: CNA 8380xxxx and CNA 8310xxxx) that show the SAME checksums for a set of programs I entered in.

Here's what I know so far:

-The calculators were brand new and have never been programmed before I programmed them.

-No variables (direct, indirect, statistics) were stored before I programmed them.

-I took care to enter the programs in correctly, but was still able to delete lines and variables and still get repeatability.

-I never ran the programs while programming.

-I didn't switch between RPN and ALG modes while programming.

Has anyone else seen this before?
I'm no expert, but I think the checksum problem is rooted somewhere in the indirect addressing.

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