I reached out to Leszek Erenfeicht to see his opinion on the gun. He is fairly knowledgeable about Polish weapons. His comments:
"A Warsaw-manufactured older karabin wz.98 (from the the time it was still written "W.98"), manufactured between 1922 and 1924, 22,000 such guns made, then the army decided between these and ex-German ones they had enough long rifles, and switched to 98AZ-style carbines ('karabinek wz. 98' - no letters), then wz. 29 carbines - only to revert to manufacturing long rifles (but then with carbine-style tangent sight, not with rollercoaster-style) "wz. 98a" since 1936 alongside both wz. 29 carbines (all arms) and wz. 98 carbines (cavalry). And the price is just eyes-popping..."
"This rifle is fishy, the s/ns are different on various parts, with 'e/77' WW2 stamp restamped with 'IEC' on the magazine floor plate" [presuming that he means the trigger guard]
I didn't catch that the letters are different. Some are EC and some are EG.