As you have alpha characters in the material number, the material number gets left aligned in the field.
hence you have to have a lot spaces right of the last digit
M210229 9001 is not the right form then, it has to be
M210229____________9001
(instead of the underscores you need to have spaces there)
so best you create internal fields for material number with a length of 18 e.g. data: HMATNR(18).
for a Space and for the plant, and then you concatenate HMATNR with SPACE and PLANT