Yes, they have and still make CNC billet aluminum backing plates for their turbos. At one time they were selling them to LE. One thing you can check would be to remove the backing plate brackets one at a time and look for these part numbers that are cut into the compressor covers' edge typically underneath the backing plate bracket. CCE44
Also, I'm not sure if Limit engraves a part number on the discharge, but you could check for that as well. The last thing you could do would be to remove the compressor cover and take the inducer and exducer measurements of the compressor wheel and compare them to the stats on PTEs site.
HTH
Patrick