Because the 123 block hole spacing doesn’t match the Sherline tooling plate’s 1.16 inch screw hole grid, each block has only a single 10-32 SHCS holding it down through a cap screw head pin:
The spring clamp squashes a pair of reasonably straight steel bars against the blocks, whereupon gentle tightening can produce
perfect Good Enough™ alignment.
You could remove the tooling plate and attach the blocks directly to the Sherline’s table with two (or more!) T-nuts and screws per block. I expect no standard SHCS length would be quite right for the distance between the head held in the block pin and the T-nut in the table slot, not to mention removing and reinstalling the tooling plate is enough of a nuisance I’d rather not do it without good reason.
Note that the remaining 10-32 clearance hole in the fixture (for the cursor hub) doesn’t align with the underlying hole in the block; the next fixture must take into account both the Sherline and the 123 block grids, as well as which block holes align with the tooling plate. Bleh!