Later. Don't want to spoil the surprise :)
So did you guess?It is in fact a picture rail fastening. Patented in 1935, which ties in well with the build date of the house which is 1937. Wonder how many more we will uncover as we work our way around the hallway!
Ha! I tried to 'cheat' and look up the Patent number, but didn't get any meaningful responses. I decided I didn't have the correct number, I made it out as 450927.
It is 430927.