Daniel, yes #591 would be the second "production" unit. There were more pre-production "Test Units" I believe 90 of them with an additional ten prototypes. I could be wrong on those numbers. Craig Carney, or Paul Bell or Tom Hoffman will be along shortly to correct me if I got the numbers wrong. BTW, forum member Tom Hoffman at one time owned #590 and Paul Bell owned prototype #409 which I believe now belongs to Tim DeLooza. Former forum member Jim Chabot owns(ed) #522.