There are several threads on Lee-Enfields and Number 4s and .303 and so forth, all "on the go" right now. Cross-check with those for more information regarding handloading the .303 cartridge for the Lee-Enfield Number 4 rifle. It's a lot of work to put together a lengthy, informative post and it's a waste of bandwidth just to duplicate it all over the place, so a bit of mercy on the guys who answer, and on the site owner, can go a long way.
This is a nice early Number 4 (that round cocking-piece did not last long in production) but it has been messed with by some shop in England trying to make it more "practical". THAT can be undone, but I think the barrel also has been bobbed.... and that can not be undone short of replacing the barrel. You have a very nice Moose rifle here, for about $800 less than buying a new one.... and it will give you a lot less trouble than many new rifles, too. The .303 will anchor anything in North America if you can place the bullet and the Lee-Enfield has been the world's best freezer-filler since before the invention of the freezer.
You can source bulk fresh cases for $60 a hundred, unfired, from Trade-Ex; they are a site sponsor, so you just click their logo at the top of this page. Their .303 brass is Partizan, made in Serbia and closer to MilSpec than anything else available.
The rifle should be marked wll on the lft side of the frame. That gives you the 'where and when and who' of manufacture.
Hope this helps.