Blue DV505a are rare.  I have two DV505s.  Neither has the cueing device, but one is black crackle and the other is like the one pictured.  My DV501 does have the cue device, but I think they all did. I only use one of the Dynavector tonearms and do not know why I keep the other two around.  I just like gadgets.

It certainly is a gadget.  I have never worked on one, never heard one either.  How do they perform?

Dynavector DV505 tonearm was awarded the Design and Engineering Award at the Consumer Electronics Show (C.E.S.) in Chicago in 1977

The award was repeated five years later, in 1982, cementing the DV505’s reputation as a groundbreaking piece of hi‑fi equipment.  The arm’s unique “bi‑axis inertia separation” technology, which separated horizontal and vertical moving masses to reduce resonance, and its combination of high‑mass lateral stability with low‑mass vertical tracking force