Just picked up an adjustable driver's backrest for my 93 FLHTCU but it came without the mounting bracket. Which part do I need for a 1993??

Here is the backrest

 

You do have the backrest mount that goes for the adjustable rider backrest bracket, and for your model year bike they were not available.

This is the under seat mount for the manual backrest - Rider Backrest Mounting Kit-52587-09. You use a spring/pin loaded insert backrest and set the height in the bracket mounting holes. Its fitment is for 1988 thru 2014 and retails for $39.95 from HD website.

This is the mount for the backrest you are showing - Adjustable Rider Backrest Mounting Kit-52593-09A. It retails for $219.95 from the HD website (this is based on my 2007 Ultra). I could be wrong but the oldest fitment for this type of adjustable backrest is 1997.

So I think you may have to swap of jury rig the expensive ARB Kit to your bike. Sorry!

 

Bummer. It was advertised as a correct fit from the seller on the auction site.
Oh well. That is a loss of $75