Mitchell acquired the rights to High Standard, and IIRC simply made copies of the older models under the HS name. Something happened later, and another company or person bought the rights to High Standard, and now has them produced in TX.
Mitchell came into the game after the High Standard factory closed in East Hartford CT in 1984 due to bankruptcy. Before East Hartford, they were made at Hamden CT (creme de la creme of HS) and before Hamden, in the pre war years (and early post war years IIRC) High Standard (then Hi-Standard) was in New Haven CT.
I don't know when the Mitchell guns were made specifically, nor how many were made, etc. What I do know is what the Citation model was. The citation debuted with the 102 series in the 1950s, in the old slant grip days. Later, the 1911 aka "military" grip angle came out, with only a few models made with the old slant style grip angle. Back then, and for many years, the Trophy model was the top of the line, and the Citation was its less fancy twin. Later the Victor supplanted the Trophy (but both were still made) and the 10X supplanted the Victor (all models still made together). The Citation and Trophy had the same sights and same features, except the Citation had a dull matte blue finish, where as the Trophy had the bright blue. The Citation also came standard with plastic grips as opposed to walnut on the trophy, and the safety, mag release, the trigger, (and IIRC the slide release) were gold plated / the parts were bright gold, but these parts on a Citation were dull. Many shooters bought Citations because they had all of the HS "deluxe" target features (until the victor) while being cheaper than the Trophy. You could think of it as the S&W model 27 vs model 28, or even the Colt Python vs the Colt trooper (at least, in the earlier years of the Trooper).
In my collection I have a 102 Citation "space gun" with 2 barrel weights and a compensator, and its a terrific target pistol. The Trophy had "improved" looks, but AFAIK everything else was the same. You could order the walnut grips, as mine has, which then left the Trophy with only frivilous upgrades in the appearance department.