You don't say if your gun is one of the Mk I or Mk II guns or not. If a 1400 Mk II - it only holds three rounds, one chambered, two in the magazine. There is no plug and the magazine tube cannot be expanded. The Mark II Model 1400 replaced the 1400 model (called 'Mk I' but it never had that roll mark) in 1968 and was discontinued in 1973 - so your gun is somewhere between '68 and '73 if this is a "mark II" 1400. There were 'model 1400s' made again in 1989 - but not the same as these early "mark IIs".