As with probably all collectibles, 'old' is good! The same is true with mathcovers, although, as we'll see, the vast majority of 'old' covers aren't necessarily treasures.
First, what exactly is 'old' when it comes to matchcovers? Matchcovers go back to the mid -1890's and, of course, have been produced ever since. That's 130+ years...longer than a person's lifetime. And that's where where the first problem arises. The non-collector measures time in relation to his own life. "I was a kid back in the '70' a cover from the '70's must necessarily be old. It's 40 years old, after all." Not true from a collector's standpoint.
The hobby defines 'old' is anything Pre-War (before December 7th, 1941), so right away that pushes 'old' back to at least 70 years. So now we're looking at the period from say 1894-1941. And, since older is better, I'd break the covers from that period into the following sub-categories:
really old 
very old
1894-1919: Practically speaking, matchcovers from this period are so rare that it's hardly worth discussing. There are some, but many collectors go through their entire collecting careers without owning one. As far as I know, all the covers within this time frame are from the Diamond Match Co., with the exception of a couple from Binghampton Match Co. Diamond purchased the rights to Joshua Pusey's first matchbook in 1892, and I'm guessing that their patent rights extended through this period. Hence, only Diamond produced any covers here. (There are a few exceptions) Famous examples of such old covers include the Piso cover (c. 1895), the Mendelson Opera cover (1896), and the WW I Knights of Columbus/Red Cross covers (1917-1918).
1920-1930: Even though there are more covers known from this period, they're still relatively rare and just about as difficult to locate and obtain. The most famous cover from this period, and the most famous cover in the hobby, is the Lindbergh cover (1927)
1931-1941: Now, we're talking! Here's where the great bulk of the Oldies covers come from. They're not the oldest; they're not necessarily the rarest; but these, for the most part, are what collectors refer to when talking about 'oldies'.
And here comes the second problem for non-collectors - Even if what you're looking at is a certified oldie, it's probably a run-of-the-mill cover. "What?! But it's an oldie!" I know; I know...but the truth of the matter is...most oldies are run-of-the-mill, nothing special covers. They're 80+ years old, and if you just want to collect 'old' covers, fine, but other than that they're not particularly rare; and they're not particularly sought after; other than age and in particular categories, they're not unique.
That's not to say they're not wanted. If a Coca-Cola collector, or a Tobacco collector, for example, happened to find an 80 year-old cover in his or her specialty, it would be a very nice find, indeed. And, there are certainly collectors who simply collect old covers. But for the most part, such a cover from a shoe store, or restaurant, or whatever, has nothing going for it other than age. The older covers that are coveted (those that might be called 'golden oldies') are those that have age plus something else...usually added rarity stemming from a rarer manumark or footer...or size. The advent of vending machines in the 1930's necessitated that matchbook covers be somewhat shorter. Ergo, those older covers (referred to as 'Talls' or 'XL's') that are noticeably longer than later issues have an extra appeal to collectors. By the same token, the size of the striker has a similar allure. About the same time that the length of the cover changed, so did the size of the striker. Referred to as 'wide-strikers', these earlier issues featured noticeably larger strikers. Also, the striker on earlier issues may be askew due to hand application. While not directly making the cover any more desirable, such a striker does indicate greater age.
Here are some of the golden oldies manumarks:
Here are some of the golden oldie footers:
Older covers sometimes need special care and handling. In some cases, the striker may have become either soft or brittle. One can normally spot this problem because the striker area on the inside of the cover is discolored. In such a case, a strip of clear tape over the inside area will help give support. On the earlier covers, early Diamond covers especially, the striker material was brushed on on top of the staple. Thus, if the striker has become at all brittle, there's a very good chance that the cover will be ruined if one tries to remove the staple. In such cases, I recommend simply leaving it as a full-book. Finally, older covers were made of thicker stock, as and a result will take longer to press flat.

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