Leland the horztonal bulbs most will have either " U " marking or " Horz" marking but a catch here some old probe start MH's still use the POMB [ Post Ortation Mogual Base ]
the reason why the POMB base it have a pin so it will only go in so far and lock in the shell socket to keep the arc tube at correct postion because the arc tube some are slightly arc shaped so given performac there.
but down to the bottom line most impoart part is read the suffix on the bulb #'s like say U or Horz or BU or BD or H/45 [ horz to 45° axis ]or other per manufacter spec called for.
Merci, Marc
