The MHL4 amplifier triode (mu = 20) was not much used in domestic radios because the MH4 (mu = 40) was better suited to the detector/first AF task. However, the MHL4 did see service in professional and military applications over many years until it was superseded by the L63, which had comparable characteristics.
The MHL4 was marketed from about 1930. It was certainly a year or so behind the MH4.
The Marconi label. The top picture shows the Type designation in the typical M-OV lozenge.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 96 mm tall.
References: Data-sheet, 1040 & 1043. Type MHL4 was first introduced in 1929. See also 1929 adverts.