I hear it can be done. How?

Hi Tele, the arm and joint are affixed to the Bigsby pin rod with a grub screw (when the Bigsby is under string tension, the screw is facing the guitar top). You can pick up replacement arms (std, chet, thick) with the joint from most suppliers and guitar shops which deal with Gretsch/Bigsby or on the Bay for reasonable prices in the States. The US made Bigsby and the licensed have a different pin rod diameter, (if I remember correctly, the licensed is slightly wider, do a google search to make sure). I'm not aware of any licensed models with a fixed arm. If you have a fixed Chet (wire) fixed arm you want the Chet style swivel, you could just buy a Chet swivel arm joint for less and use the same arm (attached to the joint with two grub screws).

Oh wow, it's that simple? Thanks Hudson! I just took the arm off, and now need to find a donor hinge, which I bet I have in a parts box somewhere!

I just checked your photo album.. if it's for the '56 6120 with the thick arm, you'll unfortunately need a new joint and arm I think... I don't have a Bigsby with that style arm, but I don't think that it can be mounted on a swivel joint and.. well swivel... I think I remember seeing that style arm in a swivel version back when I was looking for a replacement for my Gent.

[EDIT]
Just did a quick google, the only thicker swivel arm seems to be the Duane Eddie '56 handle... which is not quite as Harley as yours.... one with pictures here : http://www.guitarpartsresource.com/trem_bigsby.htm
But did find this in google pictures:
http://i133.photobucket.com/albums/q73/badrat32/BigsbyHandles.jpg
so they appear to exist..

I also paid a lot less for the Chet arm than listed there, so shop around and compare..

I think my question is the same as Tele295's. I would like to fit the Chet wire arm to my 5129 if possible. Will it go straight on or is there an adaptor etc i need to get to make it happen?

You will need to ream the arm joint out a little bit then it will works just fine, I have one on my 5120.

Thanks pickfish, don't suppose you could post a close up photo ???