Some mast lights are dual purpose - anchor and steaming lights, with two bulbs. If that's the case you DEFINITELY replace both while you're up there. In fact, any other bulbs you'll encounter on the way up (e.g. deck light) you should also consider replacing in the interest of one climb, multiple tasks completed.
When I did this with my 2009 H36 I took some photos as close up as I could get from shore and then matched those up to photos on the internet. Mine turned out to be an Aquasignal. I couldn't find any other good source that could tell me exactly what was up there, and felt better visually confirming. Fairly Easy to find the light fixture by comparing pics online, and then buying the right bulbs - AND CRITICALLY - understanding how they come apart and what tools you will need BEFORE you go up. That took some digging but it was time WELL spent.
Also, take a camera of some sort up there. You'll want lots of photos of what you have at the top of the mast, what shape its in, what brands things like antennas and windexes are, and how the top of your mast is configured with hardware and lines. Trust me, you'll be looking again later. Plus its a cool chance to take a rare opportunity to get some photos of your boat from overhead!
Good luck.
