Yes it's built with Reynolds 531 Professional which is the later version of Reynolds 531SL. The fastback seat stays are double tapered (thin fat thin) which was a feature of the new 531 Professional tube set. The frame may have been repainted and the later decal wasn't to hand or may have been a special request by the original owner
. There is a Panasonic Team bike in 531 Professional which has both decals at the request of the owner that I've seen so anything is possible. Is the frame finished in flat or pearlescent white?