At the left rear corner of the block, sort of hiding below the carburetor heat shield, there should be the attachment point for the tachometer drive cable (looks just like a speedometer drive cable). All MGA and early MGB with the 3 main bearing engine had this mechanical tachometer drive. Later MGB, starting in 1964, had the 5 main bearing engine and no mechanical tach drive. If the car carries a 5 main 1800 engine it will have to have been switched to electric tachometer. There are also some 1500 and 1600 and 1622 engines originally used for other applications (such as the 1500 MG ZB Magnette) that may not have the mechanical tach drive.

Addendum January 22, 2020:
The following cars also had the mechanical tach drive on the Austin B-series engine:
Riley 1.5  -  1489cc - 1957-1959 - 39,568 made
Riley 4-68 - 1489cc - 1959-1961 - 10,940 made
Riley 4-72 - 1622cc - 1961-1969 - 14,151 made
These engines were originally painted light green.