The ‘Uvals’ array should have the information you want:
D = load('cat cat matlab2.mat');
a = D.a;
non_empty = cellfun(@(x)~isempty(x), a);
[r,c] = find(non_empty);
for k1 = 1:max(c)
idx = r(c == k1);
vals = a(idx,k1);
Uvals{k1} = unique(vals); % Unique Values
end