One nice addition for your thematic stamp collection are Stamp Gutter Blocks and Strips or Gutter Pairs.Originally markings in the gutter of a stamp sheet were there to aid the printer by showing the colours used in the printing of the stamps. This started in the 1960s in Australia, with new multi-coloured printing techniques. Later when marginal were no longer necessary margins were used for decorative makings related to the issue. These can add extra information or interest for any thematic collection.
