Before the second world war Mayer and Grammelspacher in Germany produced air rifles and pistols
under the Diana name.
After the war the machinery was appropriated as war reparations and Millard brothers in Scotland started
producing pistols and rifles under the Diana name, hence your model 2 made in great Britain.
When arms production recommenced in Germany M and G started to produce rifles and pistols again as before
but were unable to use the Diana name as before it already being in use.
They then called their rifles and pistols 'Original' hence your Original 2 to denote they were the original manufacturers.
If you look at the bottom of the grip (butt) you should find if undamaged some stamped in numbers representing
the date of manufacture ie 4 60 for April 1960 etc.
As you rightly observe apart from minor variations they are essentially the same.
ATB Chris.