It is coded also as 16E22791. I have 16E which is the RAF Vocab for Mechanical Transport Electrical Equipment. It is a weighty book of over 600 pages it is dated 1954 & unfortunately does not go up as far as 22791.
It has quite a few 24v dynamos the nearest to G7B24-7X is G5524-2X used in some AECs, Scammells & Thornycrofts.
I have a CAV dynamo booklet unfortunately it only goes up to G7A24 but the 24 bit is as one might guess relates to the voltage output. The suffix I think relates to the enclosure. The ventilated G7A24 is rated at 55A & the enclosed version at 40A.
Sorry I can't pin it down but that looks like the sort of application & being ventilated like that is not an AFV sort feature.