In response to MitchW's question in his "generator problem in three acts" thread, I've come up with this info on instrument bulbs.
Depending on year and 105 model , there are two types of bulbs. Referring to the pic, the bulb on the left is an OEM bulb made by Osram part #3898. It has a 2 watt rating and it's size is T-3 1/4 minibay. I've yet to find a source for these. The bulb next to it, however, is a direct replacement. It's a Wagner #11009 and should be available thru any autoparts store.
The three large bulbs fit the Veglia Borletti gauges in my 68 1750. The first large bulb is again an Osram but with part #3796 and a 2.5 watt rating. Haven't found these either. The 2nd large bulb is a trade #53 sized at G-3 1/2 but rated at 1.7 watts, slightly dimmer than the OEM Osram. The bulb on the right is a trade #1836 but this isn't listed on the chart.
What is listed though is a trade #1445 (BP2) G-3 1/2 minibay rated at 2 watts. Sounds like the bulb I'll be using.
Trade number bulbs can be had at your local autoparts store.
I'd recommend changing all the bulbs at the same time. Not only will the gauges be lit uniformly, but if you don't, you can bank on the bulb next to the one you just changed will burn out next week.