Aqua Signal 25 are the main type for mast mounted lights. Check your steaming light or stern light , it should be the same size Generally these are festoon type bulbs. nd you need an all round one for the anchor.
1.75 inches long and 10 watts or the equivalent for the 25 model . This fellow sell good quality LED bulbs. The cheap ones are 12 v and will burn out when exposed to the 14 v we occasionally have. Most of his are 10 to 20 V range,.
If you want to change the unit entirely I have been using this twilight/anchor light for nearly 20 years
Be careful climbing the mast, you're a braver fellow than I !!