I've seen the movie MANY times (it was, and perhaps still is, 'required watching' for subalterns in the Seaforths of Canada and we used to puzzle over the tartan and cap-badge as well.

The cap badge looks a wee bit like the old Cameronian badge, and I wonder if the director deliberately didn't give the viewer a clear look at the badge throughout the film, knowing that if he used an extant badge then the 'other' regiments would hold up any and all of the foibles and shortcomings and sniff that it was 'typical of that horrible lot' and the legitimate wearers of that badge would cast about for windows to smash and heads to butt. (ask any former member of 1st or 2nd Bn Black Watch of Canada about one night in a certain valley in Ontario!)

Likewise the tartan. In some scenes it sort of/kind of looks like Cameron of Erracht, but less so in other scenes. I suspect that the producers chose a sett and had the kilts made up. Call it MacBBC, perhaps.

The British film industry used to be very careful about accuracy regarding films featuring any aspect of military or Empire - there were too many people in the audience who would laugh them out of court if they got it wrong.

There was a famous scene in "Ruggles of the River" (I believe) where the Native Bearer (incidentally played by Jomo Kenyata, who later became Prime Minister of Kenya) makes an epic run through the enemy lines, only to collapse at Bwana's feet and gasp out his message. the scene was supposed to be fraught with tension, but every time they showed it in the UK they had to stop the film until the laughter died down because enough people spoke Kikuyu to interprete his lines as; " I really don't think that I'm being paid enough for this part!"

Hence a tartan and badge that wasn't quite like any real regiment's, so the professional fault-finders like me would just shut up and watch the film!

Incidentally, the movie "Drum" (starring Victor MacLauglin and Raymond Massey) features the Gordon Highlanders.

Amalgamations are never popular with the soldiers, and the Cameronians/Scottish Rifles were no exception. Almost without exception after the Cardwell Reforms in 1881, the battalions of the 'new' regiments continueed to refer to themselves by their old names and numbers, answering only to the new names in official correspondence.