One Stop Oak Framing Shop in Wiltshire
The M79 Sledge Head is equivalent to a 2 lb sledge hammer and is another interchangeable head that fits both the M1 and M4 handles.
Spanner to change head is available here
Dark Knight Martinez Tools 12oz M4 Titanium Handle, Steel Head Framing Hammer
Dead on Tools 22oz. Milled Face Steel Hammer
Dead on Tools 22oz. Smooth Face Steel Hammer
Estwing Ultra Framing Hammer Leather Grip 540g (19oz)