I just received this pen via an online auction and I'm trying to pin down the identity. It's a Sheaffer, cartridge/squeeze converter, "Gold Electroplated", "Sheaffer Made in U.S.A.", nib is inlaid 585 14K gold. I think I have it narrowed down to either an Imperial 727 or 777. The differences between the two models isn't clear to me. Any help would be appreciated.