This is the first version of the venerable Datamath 2500 that was sold in numbers (see Version 0 here). It is identical with the version 0 but the case has a different color scheme. However the charger and internal structure are identical.
The version 1 is quite rare because it was replaced quickly with later, cheaper versions (see Datamath Museum page for history). However is nowhere near as rare as version 0, looking at serial numbers which has 5 digits it seems that quite a lot of them were produced (this one has S/N 99391).
The calculator uses 6 AA rechargeable NiCd batteries which need replacing; I chose to replace them with an encapsulated battery to avoid future problems because of leakage.