It's uniques because it's not listed in the Standard Catalog of S&W.
I like it because I'm a huge fan of 3" S&W's particularly the 3" model 13!
BTT = Back To Top also referred to as bump to top.
I'd like to know what you find out as far as what that product code means. I'm still a relative newbie having only been at this for 5 or 6 years now. I'm sure someone here may be able to help you figure it out.
This forum has been a gold mine of information for me.
Here's a couple of mine:
13-4
13-3 & 13-4 with 625 Mtn Gun
13-3 with Marlin .357 levergun