Well some brands have diff manufacturing techniques than others so it may be why sol asked about a specific type.

Would a standard M59 universal position lamp be fine or should I find a specific type ?

are the lamps in the fixture vertical or horizontally mounted?

Sorry for the confusion : I wasn't clear with my question. What I meant is would a standard open-rated M59 universal burn lamp be fine in a base-down configuration in a fitting that is not completely enclosed. There is a gap between the reflector and the ballast housing. The original mounting position was base up, but seeing that I intend to use these upside down, then it would have a base down configuration so an open rated lamp might be the only safe solution. I hope to use standard lamps due to cost, but things are what they are and if I need open rated lamps, then I will have to find some.

I agree on using the open air rated lamps.

The "Open fixture rating" is usually restricted for just certain operation positions only (usually the lamps are universal burn, but the open rating is just for vertical), so make sure the bulb is open rated for the base down as well.

Medved : That's what I thought ; I wanted to be sure about it. Thanks.

I agree with Medved. I Know more about merc vapor or fluorescent though.