Most units like that are between 37 and 40 feet.
By running the trans serial number here http://www.allisontransmission.com/p...y-transmission
It shows the MT643 transmission. Good trans, just no overdrive.
However it has 4.78 rear gears to make up for the lack of overdrive.
For reference, My 8.3 powered bus with a MD3060 overdrive trans has a 5.29 gear ratio.
Looks to be in great shape. They don't want a bus full of prisoners breaking down, so repairs were likely kept up to date till the end.
The window bars give you a head start with building a apocalypses bus. Lol.