Buckley's Buckler is an enchanted Buckler sold at The Friendly Arm, but only in the Baldur's Gate: Enhanced Edition version. In addition to the normal Buckler properties, it offers +1 to Constitution so long as it is worn.
|“||A rectangle of mammoth hide forms this small shield. No amount of cleansing can dispel the pong of decay from this poorly tanned device, yet somehow the malodorous shield fortifies its wielder.||”|
Characters with a Constitution score of 20 begin naturally regenerating hit points at the rate of +1hp/turn (1 turn= 1 minute).
Since this shield is available at The Friendly Arm, this makes natural regeneration a possibility after the Prologue, during Chapter 1.