Our researches have drawn a blank. This looks to us like a death mask, or Churchill with his eyes shut which seems unlikely. Faces on keystones are common: most allegoric, some commemorative. We collect the latter type but the subjects normally look, erm, awake.
2019: We were contacted by Djo who suggested Beethoven and directed us to the life mask taken in 1812 when he was 42. The resemblance is remarkable and convincing and we are very grateful to Djo for the information. Why this mask is on a keystone on a house in North Kensington, that is still a mystery.
Site: Life mask on keystone - Beethoven (1 memorial)
W10, Chesterton Road, 7