2100 RPM is indeed approved for flight at 65% power or less - Refer to the Power setting table in the Pilot Information Manual.
55% Power at 2100 RPM is obtained at the following Manifold Pressure settings;
SL 22.9
1K 22.7
2K 22.4
3K 22.2
4K 21.9
5K 21.7
6K 21.4
7K 21.2
8K 21.0
9K Full Throttle
The book fuel flow figure for 55% is 8 US Gal Per Hour
The aircraft definitely flies at this approved power setting.
Note that the above heights are Pressure Altitude and ISA
Regards,
DFC