Jo, Jon + Jno are all very common abbreviations for John

Jas = James - the catch is that in many hands 'o' and 'a' are very similar and a trailing 's' or 'n' may not be easily distinguished

Ro = Robert

Matt is generally Matthew or Mathias tho there is usually a trailing 's' or 'w' but often these are in small letters and difficult to tell apart in photos

Jos = Joseph (an unusual name in Manx context) or possibly Joshua