DC Solar is what happened.
I got mine for $1250 each, shortly before Covid.
Today I see them for $2900 each on eBay. Sorry, you'll have to pay that for "each" not for "pair".
I think some went as low as $500 (maybe the 5kW model) at original auction.
Only difference between 5048US and 6048US seems to be single large battery lug vs. dual smaller lug, and better cooling so 5750W continuous vs. 5000W. But with AC coupling, SI is only (heavily) used intermittently in most applications.