Class 423?
Open Floor if correct
So... we're looking for a four car unit of which four complete sets have been preserved, it's SR and was introduced in or shortly after 1956?
The only other class I can think of might be 411, but surely only three four car sets are preserved ( plus one of the three car examples ), one of which is a BEP ( only 75% a CEP )?
Anyone can have a go now!