I don't have my reference books at hand but the Masonite clad version probably started to appear in the late 1950's and lasted till the early 1980's (end of loco hauled passenger services). The earlier paneling lasted into the 1960's, possibly into the 1970's though there is always a chance one survived till being scrapped.
There is frequent confusion, I think expressed above, between Masonite panelling and plywood panelling.
Basically, the Masonite was narrow strips appllied over the original panels or tongue in groove boards with alumimium half round strips covering the joins.
Plywood usually replaced the original panelling, and while some early applications had panel joins, later applications used single sheets covering a section of the side (between doors, for example) or forming the whole side. Eventually some EHO vans were fitted with a one piece plywood shell.
One way of telling Masonite and plywood panelling is that Masonite cars usually retained the original curved lower body, known as "tumblehome". To my knowledge, all plywood cars had flat lower sides, and some had sliding doors, not possible with the tumblehome retained.
However none of the earlier brake vans had flat sides with tongue in groove panelling. This feature was introduced with the VHO and MHO vans, which were built with sliding doors. It is possible that the KP mail vans always had flat sides, since they always had sliding doors.
I think Masonite panelling was introduced in the 1950s and was used on LFX, CX and BX cars where the small panels and numerous doors did not require panel joins. Very few side door cars, mainly HCX branch line composite cars received plywood panels and were modified with flat lower sides.
Masonite was not waterproof and distorted after some time in traffic, and was superseded by marine ply which was effectively waterproof when correctly painted and maintained. Many EHO vans were equipped with plywood sides.
IHO 1757, illustrated on P165 of Coaching Stock Vol 1 shows the narrow panels and cover strips typical of Masonite panels alrhough this might be an early plywood modification. EHO 641 on page 161 shows the later plywood design with no panel joins visible. This particular car was built with domed ends bur was rebuilt with a slightly lower roof with flat ends. 641 was rebuilt in 1963.