I really think what you have is a Professional from the early to mid 70s. The wraparound seat stay puts it in that period. All the Mistrals that I have seen have eyelets on the dropouts. The lack of eyelets are an indicator that it is a Professional.
Also, the geometry seems to point to that frame being a Professional. The steeper seat and head tube angles point to a racing geometry. The fork has a larger rake than most of the Professionals I have seen but I have seen some with that amount of rake.
Professionals of that period had a variety of chroming schemes. Some had fully chrome forks and some did not. Some had chrome on the seat and chain stays and some did not. Some had fully chromed dropouts and some were partially chromed like yours.
Most of the Professionals from that period also did not have cable guide braze-ons. The chrome Campy bolt on cable guides were usually what the professionals had.
The paint is definitely a respray. The decals are very similar to what my mid 80s Professional have so they are not correct. The Reynolds 531 decals are not from the correct period either.
I am almost positive what you have is very nice steel racing frame from the 70s. I have a little a bit of knowledge about this because this frame is on my holy grail list.
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Holdsworth Professional