I'd lke to add a couple of things I've learned from replacing the casters on 4 or 5 SSPs.
1, the shorter, old style caster insert holds the aftermarket casters better than the longer, newer style caster insert.
(When did they change the caster insert from the shorter to the newer style?)
2, it is easy to drill out the old style insert to have drainage holes like the new style. I think I use an 1/8" drill.
New Style on the left: