Well ... I think it looks like D is a spinnaker halyard and C would be the halyard for the head sail. The questionable part is that the swiveled block, which is normally used for a spinnaker is usually ahead of the forestay.
I'm making this assumption because it looks like you have 2 sheaves for internal halyards. Your comment about the mainsheet halyard seems to indicate that the aft end of the mast is where the swiveled block is. But the mast head makes it look to me like it is the front side of the mast. I can't tell but it looks like the shape of the mast is symmetrical.
So, here is what I think ... The forestay may be rigged wrong. The block for the spinnaker halyard should be mounted to the front-most pin. The forestay should be attached at the pin where the spinnaker block is now (it should be aft of the spinnaker block). It would actually be better if the spinnaker block were mounted to a crane, but that's a separate issue. It looks like you have 2 internal halyards, one for the head sail and one for the mainsail (shown under the arrow of A). B would be the mounts for a VHF antennae and A looks the penetration for the COAX cable, which should run inside the mast and penetrate the coach roof either just outside the mast at the base, or possibly underneath the mast thru the center of the foot. I assume you have a coach roof-mounted mast.
Looking at previous comment, it does appear that the shrouds have to be attached either at A or B. I don't recognize those mounting features. What's on the other side of the mast? It may also be feasible that A is a slot for the spinnaker halyard to run internal (but not likely based on the position on the side).
If I have the front and the aft edges of the mast mixed up, you may have to clarify. More description may be helpful.
The closet to us is the aft of the mast which is oval. It has the main groove. Your right about A, I found the two cables with the "T"attachments.
The RF antenna lead must be simply taped to the mast, unless it's passed through the inside. If so, a connector should be close to B with the wire visible and ready to contact the antenna when clamped, but there is none. Guess I'll need to do more searching. (Just now while examining my cables, I noticed two "U" swivels for my turnbuckles are missing, probably spun out of the fitting, Drat!)
On the other side of the mast at the same location there's only the sister oval as you stated.
OK. Tallying my cables I have the 2 spreaders, the 4 central cables two per side which attach from the hull to mid mast, and finally the 2 stay cables, back/front which connect to the stern traveler and prow respectively. I'm left with an extra cable with a "T" section. Could there be one that runs from the mast head to the end of the boom, that is, the end furthest to the gooseneck, or should that be a rope halyard setup?
It could be a spare too.
D is at the rear of the mast and it would ride very near the sail if not against it judging by the pulley location.
Anyway, best I just do some sail setting on my lawn and find out what's up.
Thanks everyone

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