BIG45-70,
I've been doing a lot of bulb testing lately as I've just kind of got started in my flashlight craze so I can give you a pretty good rundown on what you should use. BTW, my Dad had a very similar light sitting around his house. I don't think it's quite as old as yours but it's old. Does your's have an end cap switch? Kind of unusual. Inside there was a 4.8v .5A bulb. Pretty sure it's original as I doubt my Dad ever changed the bulb. I just tried to open it to look at it but couldn't screw off the bulb holder to try to get more details. If you want I can get it later.
As to what bulb to use, there are three that I have found that are very similar. They are a Mag 2 cell magnum star (xenon), a 2.4V/.93A xenon and a 2.0V/1.2A xenon. If you have an Ace Hardware nearby you may be able to get all three. The Mag 2 cell is probably the best thrower and the the 2.0V is likely the brightest, although all three are close. The 2.4V/.93A is the cheapest bulb and can be shipped to your local store for free if they don't have it in stock. Also, the 2.0V is an old bulb and I don't think is available anymore but I found them in a couple stores. Your at the bulbs limit with 2 fresh D cells so I don't think the life will be that great. If you don't have an Ace nearby, I know a site with cheap shipping for the 2.4V.
You can also use a krypton bulb of 2.4V and .7 or.75A. These can be found at any hardware store. They'e not as bright (although difference is minimal) and the light is yellower. I prefer the Xenon.