It's definitely not Canadian market, as our New 105 model was the DX-4000, same as in the USA. I'm wondering if this isn't a frame that has been rebuilt with New 105. By 1987 I would have expected a pump peg and dual bottle boosses at this level, but I don't know if the Japanese market had the same expectations. I'd also have expected a Unicrown fork. However, most telling is the headset which does not appear to be New 105. When rebuilding a bicycle using a donor parts, it's pretty common to retain the orignal headset.

Many of the late 1980s Panasonic had the serial stamped on the front of the lower head lug, so check that location. Lacking that, check the steering column when you overhaul the headset. There's a good chance that it's a off-the-shelf fork, stamped with a manufacturer and date code. Tange was their preferred supplier in the 1980s. If it is a Tange tubeset, the seat post diameter will help a bit but not much, as most 1980s Tange tubesets used a seat tube with a common inner diameter.