I am sorry for what I imagine is a really silly question, but I have an RBION that is missing one of the 555 bulb bases that twists in/out - I've got plenty of replacement bulbs but none of them come with this base. (It looks like the brass bottom in this image: https://www.pinballlife.com/images/products/blue_44_47_555_bulbs.jpg)

I haven't had much luck searching - is this a part that is available for purchase somewhere? Thank you in advance.

I took a look at the bulbs and am embarrassed to say perhaps they are meant to be 47s. They are not documented in the lamp matrix for whatever reason so I assumed they were 555s like everything else and needed a socket. (They are the row of lights just under the glass and above the temple as shown here: https://images.pinside.com/4/6a/46a4c2ebf9659a74d044db05174176c6634bce3a/squared/mediumthumb/46a4c2ebf9659a74d044db05174176c6634bce3a.jpg )

I will pick up some 47s and some of the sockets just in case. Thank you both very much for helping a newbie out!