The victim, a man in his thirties, was seriously injured in the unprovoked attack, which occurred on May 10 at around 1.30pm in The Oval, Seacroft.
He was taken to the hospital with a broken eye socket and a broken jaw after being assaulted by two of three males on two mopeds.
The suspect who wielded the hammer was riding a white moped while wearing an aqua blue helmet.
Investigations in the area have turned up CCTV footage of the suspects at a nearby gas station shortly before the attack.
Anyone who knows who the man in the distinctive helmet is, or who has any information that could help identify the suspects, is asked to call DC 3198 Ellis at Leeds District CID on 101, quoting crime reference 13220251006, or go online to www.westyorkshire.police.uk/101livechat.
Anonymous tips can be given to the independent charity Crimestoppers by dialling 0800 555 111.
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