When you look at the base of the empty cartridge cases in your collection, you will see a variety of letters and numbers.
These will tell you all sorts of things, depending on the case in question. Invariably they will indicate the manufacturer, date of production and calibre, although there will be many variations.
In order to unravel the mysteries of headstamps, you will find much information on the internet, in specialist articles and books and, of course, by interrogating your father, guardian or another sympathetic member of the HBSA.
But beware, searching for rare headstamps can become a very serious disease, so good luck in your research!