By 1925, when the Electrification was extended to Rickmansworth, the four platforms (the present 2,3,4and 5) had two sidings at the London end of platform 5 for Electric Locos to stand and a siding at the Country end between platforms 3 and 4 for the steam locos to stand. The siding at the country end of platform 2 appears to be the remains of a Bay Platform for the Uxbridge Line shuttle which terminated at Harrow before traffic had built up on the branch. In 1948 the present Platforms 1 and 6 were built at the outer sides of Platforms 2 and 5 to accomodate the widening to six. tracks.
According to a 2004 article in London Railway Record, construction of Platforms 1 and 6 (along with the overbridge) started in 1935 as part of LT's New Works Programme and completed in 1943 but installation of the tracks were delayed by the War. The new platforms opened in 1948.
The station opened in 1880 with Up and Down lines served by two side platforms. Doubling of the tracks to 4 in 1908 required modification of the previous Up platform into an island platform and a new Up platform. The two side platforms and island were linked by a new subway. As you say, the present platforms are made up of a combination of construction from 3 different periods in history (Platform 3/4 partly dates from 1880).