You should be able to see if any of the lines are crossed inside.
It might be good to have a helper pulling the different lines while you are looking inside.
If you have two line reef - I doubt there are any balance blocks there.
The blocks you see are most likely to be purchase for the outhaul.
Considering that there is no winch between the attachment point of the vang at the lower portion of the mast and the goose-neck, the internal block is certainly to allow a 2:1 purchase on the out-haul which I suspect is the line exiting the underside of the boom on the outermost sheave on the port side, below the goose neck, and wrapped around the cleat on the boom. That would be the only way one could get a sufficient purchase to out-haul the foot of the sail in any wind.
The three remaining boom sheaves, fore and aft, would be for the reefing lines. With the out-haul on the port most side, the reefing lines should be attached to the boom directly below the reef cringle. While a loop around the boom secured with a bowline would be preferable, considering the stack-pak, that may not be possible unless there is a slot beneath each reef cringle for the reefing line to pass through. If not, you can tie the reefing line(s) to the bales that slide along the bottom of the boom, sliding each directly beneath the reef cringles. Then pass the reefing line up, outside the stack-pak or through the slot if one is provided, through the reef cringle and then aft, down and around the boom sheave and then forward up and over the matching sheave at the goose neck and down through the exit slot on the under side of the boom.
To pull the reefing lines, one can position messenger lines with a metal tape measure by extending it through the boom from the goose neck end to the outboard end. The tape will be stiff enough that it should slide through the boom without difficulty (I've done it on my boom which is over 17' long so on yours, at about 12' it should be pretty easy.) If you feel you only need two reefs, use the starboard most set of sheaves on the boom for your first reef and the next inboard set for your second. Again, the reefing lines should go from the boom, up to the reefing cringle (on the port side of the sail), through the cringle and down on the starboard side of the sail to the outboard sheaves and back through the boom to the inboard sheaves. There they pass over the tops of the sheaves and down through the bottom of the boom where there should be small locking cams that will hold the lines in place once you release the tension on them as you are reefing.
Do not discard your topping lift unless your solid vang will support the boom and associated rigging while you reef. If not, you'll need the topping lift so that all of your effort/energy goes into down- and out-hauling the sail itself and not also supporting the boom and it's gear. When reefing, ensure you down-haul and secure the associated reef cringle at the luff before you start working on the leach cringle. Then you can tighten the luff, ease the topping lift and be on your way.