BOAC, from the current Boeing 737 Flight Crew Training Manual:
Sideslip only (zero crab) landings are not recommended with crosswinds in excess of 17 knots at flaps 15, 20 knots at flaps 30, or 23 knots at flaps 40. This recommendation ensures adequate ground clearance and is based on maintining adequate control margin.
Note ; Reduce sideslip only (zero crab) landing crosswinds by 2 knots for airplanes with winglets.