Steering Universal Joint Interchange List of part numbers from various sources, equal to BMC part numbers for MGA and MGB through 1974.

People have been asking, so here it is.
Generic dimensions (in inches) for the steering U-joint are:
    Bearing cap diameter = 0.750-in. (19.04mm)
    Width across caps = 2.046-in. (51.98mm)

The most common numbers on the internet are GMB GU300 and GKN HS119, and other brands cross reference to these numbers. There are lots of oriental manufacturers advertising "all brands or plain boxes". The common British brand is Unipart GUJ200.

Steering U-JOINT PART NUMBERS:
17H3873 or
GUJ200  BMC, British Leyland, Rover Group
 263-260   Moss Motors - Aftermarket
 263-265   Moss Motors - Hardy-Spicer
GUJ200   Rock Auto (cheap)

  BMC and      | 17H3873, GUJ200
British Leyland|  
and Rover Group|  
DAIMLER		 C5246
Deepreet	 ASL076, ASL077
GKN		 HS119, HS120
GMB		 GU300, GU350
Hardy-Spicer     HS119
Italcardand	 050.910.000
JAGUAR		 C5246
JENSEN		 CT311
Lobro		 U30, U030
May		 GU300
MCP		 030
Meritor		 MCP194, MCP194/S
Quinton-Hazell   QL301
Uni Cardan	 MCP030
Unipart		 GUJ200
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U-JOINT APPLICATIONS LIST (for the same part):

DAIMLER	S Type 3.4,3.8,4.2, 1963-68
GUY	Big J Series, 1970-77 all
JAGUAR	XK140, XK150, 1954-61
	2.4 Mk.II, 3.4 Mk.II, 1955-67
	3.8 MK-II, 1964-67
	3.4,3.8 S Type, 1963-68
	240 2.4, 340 3.4, 1966-69
	420,420G 4.2,3.8, 1963-1968
	E Type 4.2,5.3, 1964-75
JENSEN	Interceptor 6.3,7.2 inc.FF,SP, 1965-1976
	Jensen GT 2.0, 1974-75
	Jensen Healey 2.0, 1972-75
Leyland	Redline range, 1961-1984
MG	MGA 1500, 1600, 1600-MK-II, Twin Cam, 1955-1962, all
	MGB, MGB-GT, MK II, MK III, 1962-1974, all 1.8
	MGB 3.5 V8, 1973-76
	MGC,GT 3.0, 1967-69