Mick,
The early gearbox case was slightly different in shape compared with the later units. If you look at the main case above the square side cover, on later 'boxes, the casting starts vertical with a radius on to the top of the 'box - on early 'boxes, the casting sloped back towards the centre of the bike.
The early 'boxes were different in that they had selector forks with spring plungers that located in grooves in the selector shaft. They also used a leaf spring as a detent for the camplate, instead of the spring and plunger of the later bikes.
By all means, take off the side cover. Inside you will be able to see the grooved selector shaft (but you may have to try selecting gears as the selector forks may hide the grooves).
Regards,
Eddie.