So sorry for the delay in responding......
I finally did get the belt back on by moving both tensioners to relieve pressure (that was a handful)
It is the old belt. I have the 5 3/4"dia. pulleys (new spindles on the deck). I'm a little confused by the S/N though.
My S/N falls between the 2 numbers you referenced. 1A267H20505.
I got the mower second handed, so I don't know a great deal of it's history. It's indicating 180+ hours on the meter (the meter is working)
Also has a little growling sound in reverse.