Full Idea
Armstrong's thesis is that recombination gives all the possibilities there are. There is no 'outer sphere' of possibilities wherein are found new and different universals alien to the actual world. No extra fundamental properties of fundamental particles.
Gist of Idea
All possibilities are recombinations of properties in the actual world
Source
report of David M. Armstrong (A Combinatorial Theory of Possibility [1989]) by David Lewis - Armstrong on combinatorial possibility 'Combinatorialism'
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.198
A Reaction
I can't grasp what Armstrong's basis would be for such a claim. I surmise that current fundamental particles can only have the properties they currently have, but I can't see the impossibility of new stuff with new properties.