GEC Monoscope advert for type 1693HKM.
Advert dated to 1955. the tube appears to be slightly larger than the RCA size which became the de facto standard. It also has a larger output connector point in the centre of the tube front.
RCA 2F21 monoscope tube data sheet 1946
RCA 2F21 monoscope tube data sheet 1954
Restoration article