XXVI means made in 1970, which was probably the first year they were made. I don't think they were advertised for sale in the US until 1971. AL-1, AL-2,and AL-3 (different levels of trim, engraving, etc) were the names given to them by the importer, Garcia (same company that made the Garcia fishing stuff). Beretta sold them in Europe (and perhaps a few in the US through other importers) as the Model 300. Does yours have the Garcia name on it?
I have an AL-1 that is probably identical to yours. (Date code XX8, 1972) I bought it in close to new condition a few months ago for $300 and a box of doughnuts. The gun shop had it priced at $350, so I offered $300. The salesman looked it up on the computer and came back shaking his head, saying "Sorry, the best I can do is $300 and a box of doughnuts." I said, "Man, you drive a hard bargain - OK, I'll pay that, but you will have to trust me until tomorrow for the doughnuts." So I bought it, and came back the next day with the doughnuts.
I like that little gun, but I couldn't shoot it very well at sporting clays until I put a rubber boot on the stock to lengthen it about 3/4" and replaced the 26" barrel with a 28" bbl off a 303. Then I was really smokin' 'um! In fact, I would consider the AL-1 with the 303 barrel to be my 2nd backup, in case both my 391's fail me. I would even put it ahead of the 303.
Two days ago I won an internet auction for a Model 300, but I don't have it yet. It will be interesting to compare the 300 and the AL-1, I expect them to be identical, since that 300 has the ribless barrel like the AL-1, not a ribbed barrel like an AL-2. I only paid $154 for that one, but it is beat up, banged up, scratched, chipped, dented, cracked, faded, stained, rusted, busted, and disgusted. It is worth more than what I paid, just for the spare parts, since many of the parts will fit all the 300-series guns. I actually shot a 391 with the AL-1 trigger group in it, and it works fine except that you have to press the bolt release button to load the second shell.
The 2005 edition of the Blue Book values the AL-1 at $165-385 depending on condition.
The AL-1 with 26" barrel is very light, so it should be good for upland hunting where you need to do a lot of walking. I think if the stock was cut down a little it would make a great gun for a small lady or a boy. They would have to use extra-light loads, though, because it is so light it will kick some.
A lot of the stuff in the 391 Disassembly post may be helpful. http://www.shotgunworld.com/bbs/viewtopic.php?t=101242