Are you sure it's a 48" deck?
There are different belts for the 42" and 48".
On my 125, which should be the same as your 123, with a 48" deck, the part number is 489397R3, which is 3/8 x 75" I believe.
I think all the 48's basically mount the same, with two pins on the back, and the two lift supports in the front that slide into the lift frame arms.
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