1) The OED has examples from 1543 and gives the meaning as ‘the large hammer which strikes first; a sledge-hammer’.
In early Yorkshire references the contexts are interesting: 1445 Willelmus Routh … camsmyth …unum stethy de ferro, vocato bighorn, unum slek trough et unum forehamer, York
1543 iiij forehamers … iiij showing hamers … ij hande hamers … ij nalyng hamers … a hake hamer with a platyng hamer, Ripley
1592 a fore hammer a hand hammer a shewing hammer, South Cave. Among the bequests of the Sheffield smith Richard Hobson in 1550 were a pare of gret tonges, a pare of little tonges, a borne axe and a fore hamber, all to his Prentise William Hawll and a stethy to Richard Fyrthe.