Small sized clamshells wasn't actually common in the USA in general in matter of fact....because a lot of electric companies continued using incandescent for lower wattages.... there are NO small clamshells in PEPCO....AT ALL! But BGE has some smaller clamshells...mostly if not all, OV-10s....Frederick....on the other hand under Allegheny Power has A BUNCH of different type of small clamshells and big clamshells!!!!!
This was actually why they started making open bottom mercury vapor fixtures, to lower the prices of them.....the low wattage MV that is....
Our old electric co used lots of small clamshells. Not many are left at all. Ihave yet to take a picture of one. The ones I remember have M-250R2s at the ends of the mast arms now. That's how i got mine. 50w HPS replaced 175w merc.
Small clamshells have been rather uncommon because in the 50s most residential streets had incandescent street lights. The most common mercury lamp at that time was 400w, lower wattage lamps were rare.
This was actually why they started making open bottom mercury vapor fixtures, to lower the prices of them.....the low wattage MV that is....