Hi Colin,
Mk1 - 3 forks have only one lug for the brake reaction link (on LH leg) and plate lugs for mounting the fixed mudguard. Also the headlamp brackets are spaced for a 6" headlamp. Very early forks had a cup and cone lower steering head bearing (with 5/16" balls) and the top bearing was a thrust bearing with 2 identical deep tracks (numbered 2016).
Mark 4 -5 forks had a taper roller lower steering head bearing and a cup and cone top bearing that incorporated a spigot to locate in the fork bridge plate. The headlamp brackets were spread to accommodate a 7" headlamp. Also they were devoid of the mudguard brackets, but had another reaction link lug on the RH leg to carry the redesigned mudguard stays. Internally, all the forks were the same, but during the Mark 5 production, heavier (parallel) fork springs were offered as an option. At the end of the Mark 3 production, the LH leading link lost it's slot that located the head on the front wheel spindle - along with a change of spindle design to a plain hex head.
Regards,
Eddie.
