This is probably part of the multi-lead cunningham downhaul system. The "lugs" at the end of the brackets are probably for mounting sheeves, through which sail control lines are attached.
Such a cunningham downhaul system is useful in bringing the position of maximum draft in the lower section of the sail forward, primarily its used to get the boat to 'almost' a dead fish' neutral helm (very very slight 'weather helm' felt in the tiller); but, at much higher wind speeds can be used to radically de-power the mainsail (instead of reefing).
The cunningham system works something like this:
The mainsail sail should have a grommet/cringle (reinforced 'hole') behind the luff about 2-3ft. high from the tack/gooseneck. A stainless U shaped 'strap' goes through the grommet and has small 'bullet blocks' at each end of the metal strap; the 'control line' connects to the strap via the blocks at the lugs.
The control lines can be adjusted while sitting on either side of the cockpit. The sequence of the 'course' of the control line is probably - from one side of the cockpit to the separate "control line bridge" (over the companionway) to a block mounted on one of the stubs on that 'mast fixture, back down to a block mounted at the base of the mast and back to a block mounted to the other stub end, back to the control line bridge, and to the cockpit on the other side. Plenty of mechanical advantage to pull the cunningham downhaul and easily reachable by the crew and/or helmsman ... plus the mechanical advantage makes adjustment easy when the sail is heavily windloaded.
Cal 20 also used a 'Barney Post' - essentially a 'pipe stub' that was mounted on the floor of the cockpit that 'stood up' about 8" ... many of the control lines 'came up through' the Barney post and thence to the control line bridge. I dont know if the cunningham downhaul terminated in the 'barney post' on a Cal20.
The alternate possibility is that these stubs will have blocks attached; but are used to control/tension the barberhaulers. The barberhauler was principally used to adjust the distance of the 'slot' opening between the mainsail and the jib (instead of using athwartships fairlead cars). After all sail set and trim to perfection is accomplished, then when on a beat one would typically pull in the barberhauler (attached to the jibhseet at the clew of the jib) until the boat reaches maximum speed and begins to point much higher than when the jiblead/fairlead is simply at the rail. The lighter the wind the more you will 'bring in' the barberhauler, the higher the wind the 'more open' the slot.
Both barberhaulers and cunningham downhauls were common on the Cal20 (and other Lapworth designed boats).
Hope this helps.
