Hi Bill,
Those look similar to a couple of old Osram-GEC British bulbs I have, but only in terms of their shape and cap. 15V bulbs are virtually unheard of here, in this country most older sets were 20V bulbs wired in 12-lamp strings for the 240V supply so I think they were probably made elsewhere.
The cap is most probably silver plated. They were offered at higher cost for lamps to be used in outdoor or damp locations since they minimised corrosion and the likelihood of and caps sticking in the holders. Silver or nickel plating is still standard practice today for high value lamps to be used outdoors, such as on railway signal and certain other projection lamps.
James.