When describing mutations, $\Delta$ usually stands for deleted sequences. In this case it means the exchange of 4 cysteine residues in the NCAM protein by serines to disable palmitoylation and the association of the protein to lipid rafts. If you search for this specific mutation, you will find the paper from Niethammer et al., 2003, which describes the mutation. Figure 2a (below) shows a schematic view of the locus and is described as following in the caption:
Mutation of NCAM140 palmitoylation sites abolishes NCAM140 raft association.
(A) Schematic diagram of the structure of NCAM140 and
NCAM140Δ. The plasma membrane is indicated by the pair of vertical
lines. The semicircles represent the five Ig-like domains. The two
fibronectin type III–like domains are shown as black boxes. The
expanded segment shows the NH2-terminal sequence of the cytoplasmic
domain and the four cysteines that were mutated to serines in the
NCAM140Δ construct to remove all sites for palmitoylation.