It is a common saying that dynamic microphones just have no maximum SPL.
This saying is not technically exactly correct but for every day use use of dynamic microphones you will not find gain limitations. At SPL levels above 160 dB some mechanical problems might occur (for example the voice coil might crash onto the magnet) but these SPL levels are very hard to find in real world.
Here at Sennheiser we use dynamic microphones for distortion measurements on loudspeakers at very high sound pressure levels. At Sennheiser we have measured the SPL and THD inside a large 2" driver/horn combination with a MD 421. The max SPL this combination was able to produce was 150 dB and the THD level recorded by a MD 421 was below 1% (produced by the driver not by the mic)
So as a conclusion we can state the MD 421 has no maximum SPL limitations for practical use. However if using dynamic microphones at high SPL levels a user should be aware that the very high voltage (up to several volts) delivered by the microphone might cause problems when connected to mic preamps.