The weapon you have is commonly known as the Model 64 Stainless Steel Military and Police, and as the name states, it's actually made of rust-resistant stainless steel (instead of nickel-plated carbon steel). If the finish is very shiny and reflective (mirror-like), then it's probable that someone polished the weapon, as the normal finish should be more of a satin/brushed bare-metal color/texture. The numerical model number is the 64-1 you found, and that, along with the serial number range, puts the approximate year of manufacture around 1974. This model could be found with a tapered barrel that gets smaller/thinner near the front of the weapon, or a "heavy" barrel, which is about the same thickness from front to rear. It has a fixed notch in the frame for a rear sight.
These are great guns, smooth shooting and occasionally very accurate, once you find the ammunition they "like." It sounds like someone might have replaced the grips (very common on revolvers), as the original grips were fairly small and would have lacked the thumb-rest (or speedloader cutout) you described; also, they would probably not block the logo on the side of the frame. If your weapon has the heavy barrel, it should look something like this one (grips will be different based on your description; these are the normal factory grips for this model):
Determining the value of a firearm can be a tricky thing, as it will vary according to the condition of the weapon, the demand in your local area, along with the economy in general. I think I can safely say if it is in "good" or better condition, it would be worth at least $250-$300 as a shooting-grade gun, and possibly more if it's in better condition. If I had the money and was looking for one, I'd pay $300 for a clean and mechanically solid model 64-1 (I don't, and I'm not). Most gun shops or pawn shops will value it much lower than that (if you are trying to sell it), as they want to buy low and sell high to make some money on each deal. If I was trying to sell this at a gunshop or pawn shop, I'd be surprised if they offered me more than $200 for it, and in most shops the offer would probably be closer to $125-$150.