The brass disk is a unit marker. Some have them, some don't. Depends on how many uneducated hands have handled it.
In my area, $300 for a Lithgow is a steal. The ones I've seen lately are running around $500. Bought mine from a private seller a few years ago for $275. They've gone up in price the last couple of years.
I also have a #4 Savage made, stamped "US Government Property", a #4 Longbranch, #4 BSA, #4 Fazakerly, and #4 Parker-Hale, drilled and tapped for scope, but did not have mount or scope. (And I know there are those who will dispute this, but it is clearly stamped.)
Goal is 1 of each manufacturer, of each model, so I have a way to go yet. Still looking for an Ishapore. Last one I saw was $650, and in very poor condition. Then start on the P14 and P17.
If you're in Central Florida, check out Dow Arms Room in Dade City. The guy knows more about Enfields than any one I've met.
If you want stripper clips, check out Dealers Warehouse out of California. Last time I bought a box he still had a couple boxes left. Price has surely gone up since I bought them. Sarco and Numrich had magazines last time I checked.
Ignore the people who tell you that you can reload Berdan primed cases. There aren't any of those primers left around that anyone is willing to part with in any quantity. I spent 2 years chasing them down. Can't find the right size for the British manufacturers. The POF and most of the other Berdan primed are Berdan Large Rifle (6.35mm). It's not worth the trouble.
Boxer primed ammo is fairly abundant and cheap. I used to buy spent cases and new brass on E-Bay, but it's cheaper from one of the mail order companies, and less of a headache.
Reloading dies are available from Lyman, Lee, RCBS, and several others. Lee is usually the lowest priced set. Military swaged primer pockets can be a pain. I have a separate decapper for those as I kept breaking the tips off the sizer. "C" presses can bend(Lee was kind enough to replace it.). You can find a "starter kit" for less than $200. All die sets are slightly different, sometimes even 2 dies from the same manufacturer due to "tolerances". Stick with 1, find the best load you can with that die set before you buy another set in the same caliber. Unless there's something about that design you don't like. Some people prefer the replaceable "decapping pins" in the RCBS and similar design. The Lee decapper is designed to "pop up" if it has an obstruction, like a swaged military primer, or CORN COB stuck in the case.
Get a CURRENT reloading manual, and follow it. IMR 3031 powder works well for me, as does AA2230, and BLC-2. The trick is to find a combination of case, powder, bullet weight and design that gives the best accuracy in YOUR rifle. Different cases have different capacities, sort them by headstamp. Different bullet weights shoot different in different firearms. Try a few different brands, weights, designs. Different powders push different bullet designs differently through different barrels. And remember, that combination may not work well in any other rifle.