If your Miracle number is really No 31 (check no stamped on the transom) then it a really early one, the first production or home build Miracle were produced in 1975 and the sail numbers reached 776 in that first year
Re your question about the tack downhaul for your Mainsail ,the Measurement Rules say you can have an" Adjustable Tack Downhaul for Mainsail : control lines may be led to any points on the hull, mast or boom"
The original Holt mast was a round section and the simplest arrangement was a with an eye at the inward end with a single block with becket fixed to it, rope goes from becket through the tack back through block and along the boom to a clamceat. Later booms have an oval section often with aa clevis pin at the inward end which Makes attachment of the block simpler
Hope this helps
John