No. It is a meaningless representation because the digit 8 can not be used with base seven.
The symbols chosen for digits need not be the usual symbols. Here is a system for base four:
zero == @ one == ! two == # three == $
Here is a number represented in that system: !$@#
Here is what that means in positional notation:
!$@# == !×(!@)$ + $×(!@)# + @×(!@)! + #×(!@)@
The base in this system is written !@
which uses the digits of
the system to mean one times the first power of the base plus zero
times the zeroth power of the base.
This example illustrates that positional notation can be used without the usual digits 0, 1, 2, ..., 9. If the base is B, then B digit-like symbols are needed.
What is the base ten representation of !$@#
?