Watch List notes:

The Robert P Ball Cabinet of Ancient Coins | Argolis, Argos, AR Diobol, c. 420-410 BC, Corinthian helmet to right rev. Corinthian helmet facing with to right, all within incuse square, 11h (BMC 29 = Traité III, 613, Pl. CCXV, 9 (same dies); SNG Copenhagen, 20 var.), an overt die break on reverse and striking splits to flan, otherwise handsomely toned, mesmerisingly so in recesses, a really bold very fine and truly much as issued, very rare.
Provenance
LHS Numismatik, 8 May 2006, lot 1035 - CHF2,000
A Moretti, Leu 30, 28 April 1982, lot 139

This very rare issue, as posited by Babelon, appears to commemorate a short-lived alliance between Argos and Corinth around 421 BC.
Estimate: $1400 - $2000