I also have an early model 67. From what I found, the flute or finger channel was not used after 1935. The protruding thumb bolt that holds the stock to the action was also recessed after 1937. As you have the fluted stock, I would say it is 1934 or 35. My gun also has British proof markings and it has the rare factory holes drilled for scope mounting.
That info on the finger groove fore-end is what I've always read too (in mags/books and online).
BUT...my dad's 67A did have the finger-groove stock so I'm not 100% convinced that the 67 to 67A transition around 1937 is right on the money.
He's had this rifle since he was a boy in the 40s. It was purchased used so we don't know 100% for sure if it's all original or not, but it was bought from one of their neighbors so we've always thought it doubtful that it was a refurbish job only a few years after manufacture.