well the mercury vapour lumiaire system been around here for very long time i belive it started around mid to late 1930's until now.
ok the mercury vapour light bulb have pretty long arc tube inside of it and the color of the light it self is bleu-greenish color and they do have very long service life many do run 24,000 hours and plus but after about 18,000 hours the bulb start to go dim over the time because the arc tube get blacked out from tugsten filment [ hope spelling is correct] .
mercury vapour light bulbs have the lowest LPW [ lumines per watts ] in the hid family and the mercury vapour have typicaly about 58 to 65 lpw compared to metal haldie bulbs they do almost double over the mercury vapour ratings.
the cost of mercury vapour lumaires is kinda low but common used in sectery lighting and landscaping which they do bring out green out nice.
but many years ago it used to be very popuar and one catalog book i have old one the biggest one they have that time was 3,000 watts
excat size i was not sure but feel bigger than bt56 bulb size wise
hope it will help some info here.