I think that was the W suffix. The plate iron was the "commercial grade" line, rated for CCS and if the W was on it, there would be a lower CCS current rating for FWB. The plate supply transformer in my 3-400 rig is (if I remember correctly) a CG301W, 2.5 KV 500 ma. FW, and 350 ma. FWB CCS. The guy who built the rig built the PA plate supply with a FWB of four 3B28s (might have been 866As but Xenons were in there when it came to me). Someone had re-wired it for FW using only two of the 3B28s and a crazy use of the primary, only 1/2 of it or something which made no sense, but I got all of the data on that transformer and put everything back to what was obviously the original intent of the builder. I run it at 2 KV.