I have a 33' masthead sloop. .... A previous owner had added a sheave for a staysil ( i am assume) as there is a padeye on the fordeck and running backstays. The crack begins at both of the lowest port and starboard screw holes of the stainless steel sheave housing, and spreads aft on either side for about 2 inches. The cause of the crack i assume is from possible corrosion ( there is minimal) coupled with the loss of structural integrity with the housing of the sheave, and the fact that the mast pumps when sailing to weather putting stress on this weak point.
Firstly, if the mast is Pumping when you are sailing to weather, you should have been using the running backs to prevent the mast bowing forward at mid span. I suspect your rig does not include forward or aft lowers (although perhaps a baby-stay forward). The runners are intended to replace the aft lowers and should be used in all but light air, particularly so if you've got a good load on the main sheet. The mast should never be bowed so much that the forward face is allowed to go into tension which is the only way such a crack could have developed. Runners are a pain in the neck but if you replace the original wire with 1/4" spectra, they are less so (been there, done that!).
From the regular shape of the cutout, I suspect that the fitting was made by the builder as few riggers or back-yard mechanics make nicely finished slots or cutouts. As previously mentioned the fitting is most likely for a sheave-box for a pole lift. If the fasteners were stainless, the pressure of corrosion around the fastener coupled with tension on the front of the mast due to bowing (i.e. pumping) from want of runners could have started the crack and, had you not noticed it, would eventually have allowed the spar to break over aft at some point.
Based upon your pictures, the crack isn't very serious or difficult to repair. One would begin by drilling a small hole at the aft ends of the cracks on either side of the mast and then machining a grove along the length of the cracks that would subsequently be filled with a full penetration weld and the weldment then ground flat. As previously suggested, a cover plate, itself slotted to accept the sheave-box, could then be laid over the front of the mast-covering the cracks on either side and welded in place with fillet welds all around. One could shape the cover plate as a "fish" (or the diamond shape previously mentioned) but that would add complication and really wouldn't be necessary. The fasteners for the sheave-box should be isolated from the spar with Tef-Gel or the like.
The foregoing repair is not that difficult and could be done relatively quickly by a competent spar-maker/welder for a few hundred dollars (excluding pulling and re-stepping the spar of course).
In future, use the runners. They were not included in your rig for no reason.