It's not quite the DL-21. It's really close and it obviously based off it, but the details are different. The barrel shroud has been cut down, the barrel itself is wonky, and the grip looks different (and much nicer) than the DL-21's.
Honestly I like this version better.
I would love to have this blaster...it looks so natural and comfortable, it would be a great "gunslinging" type of weapon.
Regardless of it's 'accuracy', I don't doubt it's authenticity. Think about how many Rebel soldier extras there are in ROTJ, not to mention ESB. I would be willing to bet this gun was worn by one of them - or even if not, present on the set as a backup or something. Stranger things have happened.