OM1n's do not have the foam rot problem. I don't know what S/N the change was made but some very late OM1 MD's don't have the foam sealing around the prism either. This change applied to the 2's also. I have modified many OM1's for 1.55V batteries by placing a germanium diode biased forward in the battery circuit under the bottom plate. The battery compartment was modified with a Nylon ring so the SR44 battery would fit. However the S625PX can be used if you want to leave the battery compartment alone.