Motor Cross South (MCS) Bicycles was started in 1979 by BMX pioneers George Esser and Rick Connery out of a motorcycle shop in Pompano Beach, Florida. The MCS business was sold not less than five times over the next twenty years and continues to exist today as a BMX components supplier: https://mcsbmxracing.com/.
In 1984 MCS applied their welding skills to making MTBs for a few years, all of which included their unique “egg beater” fork. The timeline is very uncertain, as the parts mix on all existing examples is very mixed, date wise. The sticker set on each example included here is unique, which indicates the years are different, but dating by parts doesn't work well. MCS literature from 1985 matches the following 1985 example well, perhaps anchoring the other dates.
1990 IBMXF World Championship in Le Castellet, France facebook
1984 AT Dominator
If we can trust that the parts are original, as per owner's assertion, then the mix of parts is: XT M700 v1 rear derailleurs (1982-83), SunTour MounTech v1 front (1982-85), SunTour LD-2800 shifters (1983-85). This, plus the older sticker set points to an older build. However, the Shimano B124 chainrings (1986-88) could be a replacement, as who would replace so many other components which are out of date?
1985 AT Dominator
SunTour XC 6300 Black rear derailleur (1985-86), Dia-Compe New Gran Compe NGC982 brakes (1985-87). However, the front derailleur is missing, throwing away all date certainty.
1987 AT Dominator
Shimano Deore XT RD-M730 (1987-88), plus M730 shifters. But why include the older front MounTech v1 derailleur (1982-1985)?