1 davedee 3 months ago Been sorting out an old box of broken lead wires, rusty guitar strings. Hiding in the bottom I found a small oblong red and grey cardboard box with the legend Marconi (Trade mark of the Marconiphone co. ltd.) Valve written on the four longer sides.On one end that opens is printed Marconi EF86/Z729 Then B.V.A. (or maybe 8.V.A.) in a circle 13/6 P.T. EXTRA13/6 is UK predecimal currency thirteen shillings and six pence......£0.68p in todays decimal . PT is Purchase tax..the fore runner to VAT.The other end only says Marconi but the inside flap says 17 MB- 560The valve itself looks brand new!Any ideas what it is for ?