In your drawing, U1 is one of two things. It might be an integrated oscillator component, although those usually have a much higher output frequency. Or it might be separate circuit that produces a clock frequency as an output.
It looks like U15 is the reference designator for the positive power supply voltage for the circuit, to tie three inputs to a logic 1. This connection disables the blanking features in the 7447.
With input D grounded, the chip will display only the symbols 0 through 7.