Bulldog Heavy Duty COLLAR-LOK Coupler. 2-5/16 ball high-profile coupler fits 50 degree “A” frame trailer tongue. Designed for utility, horse/livestock & construction. 12,500 lb Bulldog heavy duty couplers are designed for utility, horse/livestock, cargo and construction applications. Bulldog Collar-Lok is the strongest in the industry. The Bulldog design features the only automatic and continuously tight ball fit in the industry.