RARE MULLARD 7025 / 12AX7 / ECC83 Goal Post "641" Code Amplitrex Short Plate Rib
I tested it on my Amplitrex AT1000 in both Fixed Bias and Auto Bias modes. It is labeled The Fisher, but made by Mullard. It has the code "641.". It was manufactured in 1957 in Mitcham. The 56% GM on one triode may seem a little concerning.