I don't believe that the steel used for the plates has any bearing on the colour of the action. It's prime purpose is to hold the charcoal against the action and lock plates. Unless someone tells me scientifically otherwise that back plates or packing plates makes a difference to the colours OTHER than just introducing latency into the cooling process.
T
Correct Tony, your backing/blocking plates act like a heat sink and retain/transfer the heat to the parts you want colored......and your side panel design also keeps the b/c against the action during the quench drop....
Cheers,
