Here is all the info written on the fixture:
Lamp watts max 175
Lamp = MER/H39
Max Amps 5.1
Caution: Before installing or operating read instruction GEH-3871.
I hope this will assist you in determining if a sodium bulb will work. And, btw, the high sodium light you describe is much more appealing to me than the "bug yellow" of the low pressure.
Thanks so much for your patience and help.
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