The Mullard Type EF36 was the original version and was designed as a general-purpose RF/IF pentode with a nominally straight (not vari-mu) characteristic. When used as a high-gain audio amplifier the EF36 proved to be microphonic so a sturdier version, the EF37, was developed from it.
This in turn proved to be 'hummy', and generally noisy, especially towards end of life so a special version, the EF37A, was made for sensitive applications. Williamson writes in 1950: 'since the (amplifier) articles were written, a modification of the EF37 has appeared under the number EF37A. This has improved heater construction giving greater freedom from hum.
All three types have the same nominal characteristics and when Mullard's finally got their design and production problems sorted they made only EF37A's and supplied these as replacements for the other two types. By the late 1950s the EF37A had gained a good reputation as a low-noise audio amplifier (eg. in BBC tape-head amplifiers) and is still favoured by some for top quality audio work.