The buckler is a small shield that was worn on the off side hand, particularly useful for protecting your (one handed) sword hand. When fighting with a buckler, the outer measure of fighting becomes significantly smaller and grappling and locking becomes easier. I occasionally fight with a buckler myself, and it really helps getting closer to your opponent.
This design of this buckler has been loosely based on a 1317 AD drawing from the "Legende de Saint Denis". The original probably has no holes, is a bit bigger (around 30cm) and was therefore too big to be worn as a cufflink (which is a 16th century invention). I have not intended any symbol to be portrayed here, other than a buckler.