The pic with the barrel in isolation and the red arrow, running down the side is an embossed number xxLxxx, x being a numeric serial and L denoting the series lock L upto july 70 R aug 70 onwards.
Obviously you dont need to go through all that to get the barrel off....
the hook I found that the spring from a clothes peg to have good strength and pliability, its fiddley but can be done in situe, place the hook between the black housing and switch housing slide it towards the key until it stops due to the casting at that point is a tiny hole within which is a spring, pull the hook towards the front of the bus and with the key in the ignition slowly jiggle it out note the number and return... (whilst its out give it a good clean)