I’m a bit confused about the location of your mystery block – is it on top of the deck? Also, is it screwed down flat, flush mounted, on a swivel etc?
The general arrangement for spinnaker sheet / guy is for one continuous sheet that goes from clew of the sail to a turning block mounted fairly well aft on the deck, say a foot forward of the transom. From here it then leads forward (either under or above the deck) to a further turning block about where the crew sits, then across the boat and back via the same route on the other side of the deck and finally back to the other clew of the sail.
Your block could perhaps be the second block mentioned above?
That takes care of the sheeting aspect, but a further complication is added as each sheet doubles as a guy when it is on the windward side of the boat. For this there needs to be some means of temporarily rigging the sheet direct from the clew of the sail to somewhere around about the windward shroud. The two usual solutions are either a clam cleat with a combined hook on the side deck, so the sheet can be hooked under it and cleated, or else the sheet is rigged through an eye which is thence rigged to a barber hauler at about shroud location. This can then be pulled tight to pull the weather guy down to deck level while the leeward one is always released to allow the sheet to lead direct to the normal block at the aft deck.
Both systems have pros and cons. The “simple” cleat arrangement is easy and uncluttered, but can be tricky for gybing in strong winds. The barber hauler makes that easier but clutters the boat up with more rope, and the weight of the string hanging on the (leeward) sheet can make it harder to get the sail to set in light airs.
I sail on a lake where we get more than our fair share of light variable winds so I tend to favour the simple cleat arrangement; if I sailed on the sea I’d probably prefer the barber hauler idea.