Easy questions to answer. Some Harvards/T-6 series were armed with either a cowl gun or wing gun – IIRC the Commonwealth ones were mainly a single wing mounted Browning for gunnery training. That explains the firing button. Secondly, the Canadian built machines or rather the majority destined for Commonwealth service were specified with a spade grip – a la the fighters on which the pilots would progress. The ‘normal’ T-6 series had a B-5 pistol type grip from the US factorys/for US service. So put simply the spade grips were sourced in for Commonwealth spec, and made by Dunlop who made the majority of spade grips.

Sadly, unless there is a serial number of an airframe marked on it- you wont be able to trace the machine it was fitted to.

TT