Congrats on a very nice Browning A5, a 20 Ga. no less. I worked at Browning's St. Louis Gunsmithing shop for many years, here is what I can tell you. Your gun appears to be a model that FN made for european sales, nearly all Browning A5's made for US sales had engraved receivers and lacked sling swivels. The 4Z is just part of the serial number as is the FN, acier is a type of steel. Sometime the marking Acier Steel is seen as well as Acier Special. The ** asterisks refer to the choke, if your gun were a skeet model the marking would be a $ sign. The recoil pad has been added, the gun left Herstal with a buffalo horn buttplate. From the crisp appearance of the Twenty and Browning lettering, your gun does not appear to be re-blued. I believe your gun was manufactured later than 1928.