The measurements given would be correct but not critical, and may be better described as, "20cm below the main halyard sheave and mounted on the forward side of the mast", also I feel that the old halyard lock RF182 may have been superceded by RF191.
So is the reason for having it way up there to lessen potential stretch in strong weather? I cleat mine off back on the cabin wall in the cockpit.
Ps Rob, having your input here makes owning a great little yacht even more rewarding
The reason for having a halyard lock near the top of the mast on the skiff rig is to halve the compression on the mast due to the main halyard tension, and this can be considerable when the mainsail is sheeted in hard when on the wind.
On the old style bigger section, masts this was not as important but with the smaller lighter sections used on a skiff type bendy rig it does help prevent breakage of the mast and distortion of the mainsail shape caused by compression buckling.
It is however more difficult to release the halyard in an emergency and is not very popular on cruising boats.