It could also be unfinished letters\number, like, it can not only be 7 but also Z, not T but I, not D\P but R etc:
:
__ _____ ____ _____ ____ ____ _____ ____ ___ _
|
/ | \
/ | | |
/ | | |
/ | | /
/ | /
\
/ | | \
| | \
/ | \
____________________ ________________
.
.