Re: Marshall 1912 - G12B?
I have an example of this cabinet. The OEM loudspeaker is referred to by Marshall as the Celestion G12 Gold.
"The History Of Marshall" by Michael Doyle points out that, in the USA, the G12 150 was used instead of the G12 Gold. The book fails to describe what differences, if any, there were between these two loudspeaker designs.
Try the Celestion website. It is a mine of information.