I suspect the only info you will find on Holland and Holland is from classic gun and antique gun dealers listing their goods from estate sales. Try a generic search for "holland and holland double guns".
H&H was famous for generations of the "Best Quality" English classic custom rifles and shotguns (late 1800's through the first half of the 20th century.
If you plan to fire that H&H .303 double rifle, please be advised that it was "regulated" to shoot point of aim at only one distance and with only one bullet weight and load. Shoot anything else in it and you will problably find that neither barrel will hit point of aim.
HTH
John