The chronology is a bit confusing to me, but let me see if I can help, subject, as always, to better info. Around 1962, Browning came out with the Nomad and the Challenger, then added the Medalist in 1964. All were basically the same, the latter being a refined target pistol type with a wood forend. Takedown was by removal of the barrel; a screw in the front was turned out and the barrel lifted off, like the High Standards of the same period, except the H-S guns used a push button. All had a sharply raked grip.
Those guns became too expensive to sell at a reasonable price and were discontinued around 1975. The Challenger II was introduced a couple of years later with what is now the basic Buck Mark design. With the advent of Browning's "buck" trademark, the name Buck Mark was chosen for the entire line, so there have been a number of variations - Challenger III , Camper, Hunter, Bullseye Target, Lite, Contour, Varmint, etc., etc.. Takedown is by removal of the sight bar screws and pulling the slide upward.
Somewhere along the line, Browning apparently was made aware that "Challenger" was a Colt-owned trademark*, so Browning dropped the "R" and the later guns use the name "Challenge."
HTH
*For the fixed sight version of the Type 2 Woodsman.
Jim