Regarding drawing B19564, Trevor says:

Sorry B19564 does not show the distance between the supports. That information is on C24586 (which we do not have a copy of).

However one can guess that the chocks are intended to fit in the gaps between the angle beams on the trailer shown on drawing B19564. If you look closely you will see three pairs of angle iron beams crossing the trailer with the leg of the angle uppermost, the outer sets forming slots 3 3/4 inches apart and 3 1/2 inches apart. The beams for supporting the boat are 3 3/4 and 3 1/2 inches wide.  Not so easy to see where the third set fit though.

On the other hand the dimensions can be found by measuring the distance between the Aft Cabin front bulkhead and the main diagonal bulkhead. The beams go one under each bulkhead.

Hope that this helps.