I agree, I've trawled all of my Met and LT books and any information is a bit vague when it comes to whether all of the Westinghouse and BTH locos were 'reconstructed' and withdrawal or scrapping dates. We do know one Westinghouse loco and one BTH loco were reconstructed in 1919 and after these two it was it was decided to construct twenty new locomotives. The BTH electrical equipment was removed and used in multiple units. Both the Westinghouse and BTH locos had gone by 1922/1923 once the Metropolitan Vickers locos were in service.
When I was researching the Met Bo-Bo for my 7mm model I did notice two different rivet patterns on the solebar depending upon the loco number which I think may indicate either a reconstructed or new chassis.
The line from Harrow to Watford and Rickmansworth was electrified in 1925, Rickmansworth to Amersham and Chesham in 1960-62.
Then onto the electric multiple units

. 1913 stock, etc, etc