A. This sounds like the early production versions of the series 2 from c1954. It would appear that the 

first 1800 of these machines used the earlier range of "40" series of valves, in common with the rest of

the industry at the time. Ferro along with everyone else changed over to the more familiar EL84, EF86 

series when the range was introduced in 1955 ish. 

There are differences with the respective series including the valve holder . the 40 series have a locating 

pip in the glass and used the B8A holder whereas the EL84 etc uses the B9A valve holder and lacks this 

locating pip. Physically they are not interchangeable and there are probably electrical and gain differences 

to. I make reference to this early Series 2 in my book "built like a battleship". Also the power connector 

on these is a flat 2 pin type which was also changed to the 3 pin bulgin when the valve range was changed.

This information kindly provided by Terry Martini. www.ferrographworld.com