How do you tell the difference between low pressure sodium and mercury vapor bulb. The old one has no remaining markings in it.
What color was the light? Sodium is sort of yellow-ish-green, and color rendering made everything look brown-ish. Mercury has a lot of blue, and appears blue-white with not-too-bad color rendering, at least compared to sodium.
You might be able to save a lot of money on electricity if you pick up a LED.