The internet says the pistons are made by Rebco and or ACL. The "RA" means it is a set of pistons (rings not included).
RA2648 = piston only (was probably ordered as part of a rebuild kit)
6KRY2648 = KRY (contains Plain rings)
6MKRY2648 = MKRY (contains Premium rings - Moly or Chrome)
TOP UD ???
TOP YD ???
TOP YC ???
TOP YK ???
TOP CF ??? - http://www.ebay.com/itm/Toyota-2F-Mo...p2047675.l2557
Still working on the UD marking...
This was the kit that included RA pistons:
I like finding similarities on the web. This guy has the same question as you with the same conclusion even though his is marked with YC instead of UD...