Photos are from Tech book pages from 1952 for overdrive unit as described in DB2 & DB2/4 overdrive conversion category page.

Our conversion is carried out how we thought the Factory ServIce Dept would have done this as a retro-fit if they had the smaller overdrive unit to hand.

The expected price is for roadtesting the car, removing the trim, carpets & tunnel, the gearbox, then stripping & rebuilding the gearbox with new parts as required (we have quoted for the most usual parts on the price) including fitting a new mainshaft & other parts that are different, fitting the overdrive to the gearbox with one of our adaptors, carrying out some minor chassis & tunnel mods & refitting the gearbox with new special propshaft & wiring it up, testing inc roadtesting & refitting the tunnel & trim.

This compares well with fitting a new 5 speed gearbox which is not period correct as few UK built cars had 5 speed boxes in 1958 or 59 let alone 1950.

More info here