Click the image for the full .PDF of the maintenance manual.
The small Mullard thyratron Z803U, (CV2434), is used in no less than 7 places, as relay drivers, in the Muirhead D-649-L/E1 'Mufax Chart Recorder'.
The 6AS6 valve is used in two (as V3 and V22, valve complement in page 18 of pdf-file) interesting functions.
This Mufax Chart Recorder was chiefly used for drawing wheather charts on 18 inch wide paper. The attached maintenance manual is dated 6 July 1965.
The picture above is an earlier version, D-649-D/A. The D-649-L is considerably more advanced.
The valves used in the recorder are: 12AT7, 12AX7, 2D21, 6060, 6AS6, 6L6GA, 85A2, A2293, QA2403 & Z803U