How to rig the mainsail reefing line

[Hozpypr]

New to me Compac 27/2 with no mainsail and rigging.Looking to rig the reefing system. Paperwork that came with boat shows 2 different reef line set ups. One shows a 2 sheave car inside the boom with 2 separate lines, one for fwd cringle and one for after cringle. The other shows a line running from mast up to fwd cringle, back down and into mast, coming out the aft end of boom over a sheave, up and through the aft cringle, then back down and fixed to the boom. On this second reefing line rig the picture shows 2 sheaves with jammers but don't know what they are for??? I'm doing this on a pnone so can't see how to attach pics, thanks