I'm not sure which lines you refer to as "extra" on the mast. The one your finger looks like it is pulling is probably the upper shroud, that should go down to the end of the spreader (looks like that is the starboard side, as I think the wire on the upper side of the mast must be the forestay), held on by a sort of u-bolt, and then down to the outermost chain plate turnbuckle on the deck.
I can't help with the backstay, as mine does not have an adjustable backstay. On mine, two stays attach to the tangs on the stern (not the embedded "bar" that acts as a stern cleat on the 23) with turnbuckles. These lead up to a pair of triangular plates, where the eye at the top of each stay is held in between the plates with a clevis pin. The backstay from the mast crane then attaches to the top corner of this triangle. Other than the turnbuckles, I have no adjusting gear like blocks.
The swivel block in your hand may be part of the topping lift. If its wire leads up to the inner (forwardmost) hole in the mast crane, just ahead of where the backstay attaches, it is almost certainly that. In that case, the previous owner probably rigged it so the running part of the topping lift goes out the rear end of the boom (through one of the sheaves), up to and through the swivel block, and then down to be tied off to the rear end of the boom. That lets you adjust the topping lift with 2-1 mechanical advantage (compared to having the running line exit the boom and go all the way to the mast crane). It also lets you remove the boom before you unstep the mast without having to pull the topping lift out of the boom (the easy part - getting it back in is the problem). You just untie the TL from the aft end of the boom, pull it down out of that block, and leave it in the boom.
I have no idea what that block in your first pic is - I have nothing like that. Possibly part of an after-market or jury rigged adjustable backstay?
