I suggest keeping the extra SI as spares, or maybe a future system.
I found the 6000TL-US-22 you would have bought.
The 10000TLUS-10 available is unfortunately is the 208V version (unlike -12 which supports several voltages.)
I see other older ones, price high enough not a bargain compared to the new model, at price you're saying. Although I see more like $1500.
The manual for -22 does mention RS-485 interface. But I don't have details on what makes it work or not with SI.
There are people using those two, though we don't know they are playing nice together.
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