A few days ago, it was looking a little cleaner and happier, if no less faded. Happily, whoever was responsible for the new coat of black paint chose to leave the sign uncovered. It is a palimpsest: one layer advertises 'for all skin ailments - Germolene'. Some words below are difficult to make out, although with a lot of messing in Photoshop I could make out the words 'the ... skin dr...' - presumably its slogan, 'the aseptic skin dressing'.
Over it, though, was painted 'This space to let - C.J. Lytle - BORO - 57 Long Acre'. Boro (part of the C J Lytle advertising agency) appear on a number of signs, and were an agency who let the wall sites. Obviously they didn't have much success in this spot, possibly a symptom of the general decline in painted advertising although Sebastien of Painted Signs & Mosaics dates the Long Acre address to the 1930s. (This would tally with the other examples of the 'aseptic skin dressing' slogan, which are from the 1920s.) Certainly, a Gillette palimpsest on Gray's Inn Road gives the Boro location as Stratford - and promises '10,000 sites'!