I have 2 identical overhead projectors with bad diodes, but the diodes have different numbers. How do I find the right one that works?
That link looks like the same question I agree.
 We really don't need a photo of the diode just the numbers will work. Diodes are rated on the ammount of current they can rectify and the peak inverse voltage (for general purpose types). The normal failure is the junction shorts until enough current is available to blow the junction open. The diode failure in this case is probably due to excessive current not the PIV, installing a diode with a higher current rating should do the trick. Make to note the stripe on 1 end of the diode and install the new one the same way so the polarity is correct for any wave shape or starting components that may be part of the circuit to get the lamp started.