> The locative, strictly speaking, comes from (*-ei jam ...
Or probably *-oi jam (>(East Baltic development) *-ei jam, or without this
trasition directly to) >(analogy) *-am jam. The problem is that we have
attestation for both *-oi (preverbs in -ai) and *-ei (in fossilized
locatives like _namie~_ 'at home').