The short top mast on the Frensi rig is called a "halyard mount".
There is a short and a long halyard mount. Which one you need depends on your Laser lower mast and which size Frensi rig you have (the Frensi or the Frensi+).
If you have a Frensi rig and have a lower mast for a standard Laser (also called an ILCA 7), then you need the shorter halyard mount.
If you have a Frensi and a Radial lower mast (also called an ILCA 6) then you need the longer halyard mount.
If you have the Frensi+ you will need the longer halyard mount and you will also need the Laser standard (ILCA 7) lower mast. The Frensi+ can't be rigged with the Laser Radial/ILCA 6 lower mast.