fuhrmann.
Bruce Hodgdon started his company by selling Surplus Government gunpowder after WW2. At first, he had a quantity of the powder used in 30-06 ammo. Not knowing what the precise powder number, he determined it burned a little slower than IMR 3031, so he marketed it as 3031 Data Powder. He later marketed it as Hodgdon 4895 or H 4895. When he sold this powder, he bought another large quantity, but it was a slower burning powder than H4895 and was for either 50 cal. or 20mm (I don't remember which). He called this powder Hodgdon 4831 or H4831. Ammunition factories do not load their ammo using a specified charge weight for all lots of powder, instead, they load it to a specific pressure, and the charge weight may vary from lot to lot. When commercial powder mills went back into operation, they used the IMR designation and blended the powder to be consistent from lot to lot, so previously published data could be used. When Hodgdon had to start using newly manufactured powder, it was blended to more or less match the surplus powder. Consequently, IMR 4831 and H4831 are basically the same, but not precisely the same. This situation similar to a powder company having to change manufacturers for their line of powders. The old and new powders are basically the same but not precisely. It is good handloading practice to lower the charge weight when a change is made and work back up watching the normal signs. This also holds when changes in other components are made.
Mike