Always wondered why they didn’t put these in a 50 watt mercury jacket?

Same, would have been cool to see the smaller size! Never saw as many 50W SON I's about, 70W was always the default.

Oh yes the 50W MV has a smaller envelope (I rly want to get one!!) At first thought it was a usual 70W internal ignitor SON-E - maybe its something to do with the arc tube length that has to be of a certain length that cant fit in a size of a 50W MV lamp envelope

50w SON I does seem hard to come by. Whats the MFD date on the lamp?

@Liam: Yes I find it is easier to find 70W SON-I and even 250W as well - the mfd date according to the very faint etch that is not visible from this pic, I had to hold it against the 30W T8 fluorescent fixture to see it properly and can make out '100Y'