Anchor light bulb type - Selden

Although Selden list masthead lights in their catalogue, lots of Selden masts have lights separately supplied during commissioning. Additionally, many masts have had replacement lights fitted during their lifetimes. There aren't too many different brands, so you ought to be able to identify your light by careful study with the aid of binoculars.
 
Anybody knows what the bulb type of the anchor light on a Mk II Selden (in mast furling) mast is? BAY15D probably (this is only a guess)?
Or perhaps a BA15D. There's no reason for an anchor light bulb to have the off set pins of BAY15D
 
I don't understand this answer.
Both are 15mm, double contact, bayonet fittings

BAY has offset pins so that it can only fit in the holder one way

BA has in line pins and can therefore be fitted either way 180° apart
 
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