Information about the introduction of SLX in 1984 is available in many sources, applies to several manufacturers - EMC was not "first in line" (it never was, the next models of tubes were introduced with 1-2 year delay - they were the most hasty with SLX)...
If SLX had been introduced in 1983 the teams preparing for the 1985 season would have had SLX bikes already in the fall of 1984 and we would have had lots of posed photos of Kelme, TeVe Blad, Lotto and Tonissteiner teams at that time with such bikes... We only have photos of bikes with Columbus SL frames, Pro SLX appeared in teams half a year later.
Proven novelties were introduced to teams within a year at most (SLX 84->85; 653 88 ->89; Max, Mxl, Neuron in the same year)
Panasonic markings are the foundation, that cannot be disputed.
The problem exists (existed) with the assumption that memories, EMC frame, photo and receipt are parts of a whole...
If it now appears that a "different bike" appears and the photo "may be from a different race" then.... there is not the slightest reason to interfere with the existing dating: P8 E9943 - summer 1984; Z1701 - late 1984.
Either the story of the "brothers" is true in every detail and challenging or it is not true in every detail