Unlike your F28, the F27 had a fixed mast (no rotation). The mast originally had one spreader and a baby stay in the front that always caught the jib on tacks. The two shrouds controlled mast bend (like on many keelboats backstay). The second (aft) shroud connects to the first shroud with a cascade of line like a mainsheet. On heavy wind you cranked the cascade to bend the mast and depower the mainsail.Nice video!
It looks like you have two running backstays rigged up. I've never seen that on a Corsair--my F-28 just has the two side shrouds and that's it. It looks better to me to see two strings per side to hold the mast up.
How are they attached to the mast? Through the same shackle as the shrouds, or a separate fitting?
As a small digression, back in 1990 sails were mostly crosscut because the fabric used was mostly dacron. Crosscut sails put the loads perpendicular to the weft and weave. To add shape the mast "pushed in" the fabric to create the belly; so bending the mast depowered the mainsail by reducing its camber.