Couldn't be a bosun's chair. Anyone who's spent any time aloft understands how something like that would force one's legs together very uncomfortably.
I also doubt it's a spinnaker tack support as the design is pretty old, probably from before roller furled sails were on the forestay and a "Tacker" was used to pull the spinnaker tack to weather as much as possible. Come to think of it, asymmetricals probably weren't used then either...
At first I also thought it might be a boom support of some type, but given the type of grommet and very small reinforcement patch I'd say it's either for a very small boom, as a type of temporary topping lift, or, more likely, a galley hip strap missing it's connection lines and snaps on either side.
I also doubt it's a spinnaker tack support as the design is pretty old, probably from before roller furled sails were on the forestay and a "Tacker" was used to pull the spinnaker tack to weather as much as possible. Come to think of it, asymmetricals probably weren't used then either...
At first I also thought it might be a boom support of some type, but given the type of grommet and very small reinforcement patch I'd say it's either for a very small boom, as a type of temporary topping lift, or, more likely, a galley hip strap missing it's connection lines and snaps on either side.