I've got two 261 blocks, a 56 and a 62 and both blocks have those same holes in them. I believe, from what I can see on my blocks, those holes are for screws to go through to hold a baffle in place to stop oil from being thrown up into and out of the draft tube. Both of my engines have a piece of metal screwed into place shielding the draft tube hole from the crankcase portion of the engine.
Joe
