Catalyzes the hydrolysis of 4-amino-2-methyl-5-hydroxymethylpyrimidine pyrophosphate (HMP-PP) to 4-amino-2-methyl-5-hydroxymethylpyrimidine phosphate (HMP-P). 4-amino-2-methyl-5-(diphosphooxymethyl)pyrimidine + H2O = 4-amino-2-methyl-5-(phosphooxymethyl)pyrimidine + H(+) + phosphate Belongs to the HAD-like hydrolase superfamily. Cof family. Extended N-terminus. Extended N-terminus.