Arbiter was one of the names under which Rose Morris sold a variety of MIJ copies back in the '70s.
I'm fairly certain that your bass is the real deal.
You may also want to take a look at this thread:
Hi all,
I've had this old bass in my studio for well over a decade, I bought it second hand in the UK. It says 'Arbiter' on the truss rod cover and it says made in Japan on the neck bolt plate.
Google only brings up a few relevant hits but I've learnt that Arbiter brand was used for Kasuga imported copies from Japan, at least this is what I gathered from translating a German guitar forum post about these guitars! I'm really going on a shoestring of info here.
Does anyone have any information on Arbiter, Kasuga or Rickenbacker copies in general? I've no idea what the value of this bass...
Play it in good health and enjoy!