7-digit plates were issued in 1922, 1924 and 1925.
Until an example turns up, we don't know if the 1922 and 1924
7-digit plates used thinner dies or a longer plate size.
1925 #29-10461 is verified with a non-serif 1st "1"; size unknown.

 
Waterloo Evening Courier  November 1, 1922, p.7
"BLACK HAWK'S CARS NOW EXCEED 10,000"
"Nine new automobiles were registered by the county treasurer
yesterday.  The last license plates given out bore No. 10025. 
The treasurer has been supplied with plates running up to 10200,
which probably will be enough to take care of the 1922 registration.
After Dec. 1, no registration for the operation of cars during the
last month of 1922 will be required.  The 1923 licenses will permit
of the operation of new cars in December."

 
Waterloo Evening Courier  November 16, 1922, p.13
"Altho but two weeks remain before the registration of 1923
automobile licenses begins, applications for license plates for the
completion of this year continue to be received at the auto license
counter in the county treasurer's office.  With the issuing of three
licenses yesterday, the total number of motor cars registered in
Black Hawk county is 10,080."