Hi Daz,
The information I have is that all three coaches served as parts of ambulance trains during the first world war. They were then converted to open type passenger coaches of various diagrams and returned to operation in 1923/24. E62461 is listed as a 54' long coach, the other two being the earlier 50' length.
As to the final part of your question, I am not aware of any kits being produced for these open type coaches. There used to be kits available from D&S for 50' compartment coaches in 4mm and 7mm, but these have not been available for some time. Similarly there were kits available from Bill Bedford, but these are also no longer listed on his Mousa Models website. Worsley Works produces some etched sides/ends for 3 types of 50' compartment coaches.