Extended headers with padding are now supported in mrcfile>=1.5.0 (https://github.com/ccpem/mrcfile/issues/50). For older versions of mrcfile just ignore the extended header.
The only example of a padded extended header I've seen actually contained useless values anyway (with a zero pixel spacing). In this case, also ignore the extended header.
mrcfile>=1.5.0
(https://github.com/ccpem/mrcfile/issues/50). For older versions ofmrcfile
just ignore the extended header.