For starters, you could get hold of a copy of "Holland and Holland 'The Royal' Gunmaker" by Donald Dallas. This book contains a wealth of information, including Serial Number ranges.

Alternatively (for a fee) Holland's should be able to give you an extract from their original records for your rifle.

For loads suitable for a double rifle , search this forum. If you can't find what you want, PM me with the bullet weight and type that you want to shoot and I will see if I have something in my records.