As I was leafing through microfilms of vintage Signs of the Times issues, one ad piqued my interest:  by General Electric, touting what they designated as '10A21' light bulbs.  As in the March 1958 issue, they were spotlighting the Life magazine "spectacular" as displayed on Douglas Leigh's 4,104-bulb (76 x 54) EPOK board on the NE corner of 46th Street and Broadway in Times Square from November 1957 to c. January 1960; these were basically reflector bulbs, with 1,020 white ones (spelling out 'LIFE') and 3,084 red ones.  Wonder if anyone round here would have any specific examples of same, or otherwise have any further knowledge.  No doubt the width would be the customary 2.625".

I've never come across a 10A21 before, but certainly up to the 1980s Sylvania made a 25A15/RFL lamp.  It was a fairly standard pear-shaped incandescent lamp but with aluminium reflector coating over the neck area, so as to increase the forward light intensity.  They were advertised as sign lamps - producing a brighter display than regular unsilvered types, and delivering a much wider viewing angle than would be attained from using conventional parabolic-reflector type incandescent reflector lamps.

I wonder if the GE 10A21 may have been similar, with a reflector-coated neck?

Whereas most low-wattage incandescent lamps were vacuum types for maximum efficacy, some of the old low wattage incandescent sign lamps were gasfilled with helium or hydrogen.  The high thermal conductivity of those gases makes the lamps terribly inefficient, but ensures that when the power is switched off, their filaments cool down more quickly than usual.  This 'rapid-nigrescence' feature was useful in reducing the blurring effect seen in particular on moving message type boards, in which lamps were pulsed on and off at high frequency to display a rolling text message.  Sylvania produced the 20A17/5 'Flashing Sign' lamps for that purpose.

The top was reflective and from there downwards was covered, so . . . yeah 25A15/RFL with aluminum reflector coating over the neck area would have been similar to both 10A21 and 20A17/5.

I suspect the A15's (in regular mode) were what were used on the Bond zipper covering the block of Broadway from 44th to 45th Street.  Likely Verd-A-Ray, at least in the '50's.  But the A21 type of reflector bulb, I also remember from the One Times Square 'zipper'.  I am going through old Signs of the Times microfilms and have learnt a lot.

Very interesting.  Having researched this a little more it seems Westinghouse made a very similar 30A21/RFL.  There is even some old stock for sale, see https://www.ebay.com/itm/153342053205  The same seller has them in various different formats.

Turns out GE, as of 1960, did make A21 reflector bulbs in both 10-watt and 6-watt (!).  I have found this page online which has quite a few historical catalogues:

http://www.lamptech.co.uk/Catalogues.htm

The 30-watt A21 reflector bulbs would have been exclusively Westinghouse's (later transferred to Philips), but GE did for a time in the late '70's make 25-watt A21 reflectors.