Are designation of voltage current quantities with upper and lower case letters universal
The designation of voltages and currents as shown in the picture you added were part of the old IEC 148 (or IEC 60148:1963) standard, which was superseded by IEC 60748-1:1984 in 1984 (current revision is 2002): these recommendations are now probably part of this last standard. I saw them for the first time long ago, in the "General" section of a Philips Transmitting Tubes Manual: below I have added few scans from a more (but not so much...) recent Philips manual.