Sorry about the multiple post, not sure how that happened.
i always thought the 501 was a the 1st iteration, the 505 the second, the 507 the toid. they have a www that gives good info on these arms. i've heard they're excellent, tho no 1st-hand experience. i was strongly considering one for my oracle prior to getting an o-l modded rb250 - i couldn't figure how i could get an auto-lift mechanism to work on the dyna arm when it got to the end of the record, due to its bi-axxial tracking design - only the last couple of inches of the arm actually go up-n-down...