So called ‘EBL31s’ sold by UK valve dealers for > £25 or more are invariably converted EBL1s with an octal valve base fitted over the side contact base. A giveaway is that the converted EBL1s still have their 3/8” top caps fitted whereas the EBL31 is 1/4”. The fitting of the octal base makes the valve too tall to be used in Ecko A22s but if height isn’t a problem the converted valves work fine as electrically, and the size of the ‘bottle’, the EBL31 is identical to the EBL1.
Generally, NOS EBL1 valves - though not commonplace - aren’t as rare as EBL31s. As I have an Ekco A22 and my (adult) son has two, only the correct height EBL31 will so. Hence, I’ve converted several EBL1s to EBL31s by removal of most of the side contact base, fitting a reduced height octal base and changing the 3/8” top cap for a 1/4” top cap. It’s a bit of a ticklish task but wholly successful with care.
This thread gives full details of what’s involved:
https://www.vintage-radio.net/forum/...ighlight=EBL31
Hope that’s of help to someone.