Nothing. My MX 5C had drop out triggers, square monoblock barrels. The significant difference was that the action was a slimmer and lighter than my MX8.Had the 'seashell' type of curved breech like the MX8, not the flat one like the MX3.
The MX3 Special was a version of the MX8. square monobloc
I bought my 5C in England and was told by the gunshop that this model mostly stayed in Europe.