Actually, there have been many bottles of beer much older than this pilsner which have aged very well and are quite delicious. British beers from the 19th century command very high prices on eBay, and are generally well-regarded, with some exceptions (i.e., the cap/cork is compromised, and they turn stale/sour). These beers share similar common denominators:
1. The beer has a high specific gravity (greater than 1.080)
2. The beer is not filtered or pasteurized. In other words, it is still alive, with suspended yeast still working.
Unfortunately, a 1948 bottled pilsner from Great Falls is likely to fail both criteria. It most probably has a gravity between 1.045-1.050, and has been pasteurized.