Keep in mind I am comparing a Carolina Knife and Tool version of the Multiplier (not actually made be Gerber, but made to the same design) and my MP400, so there is a chance this doesn't apply at all.

OK, first of the pivots on my Multiplier are much smaller than the pivots on my MP400, so it would need drilling out. Also the 'flat' where the spring/lock meets the blade is not long enough on the Multiplier so that would need re-shaping. Other than that it does look like it might fit, but without removing the blade from my Multiplier it's hard to say for sure.
If you are just looking to add a plain edge blade to a MP400, you might not know that all the blades/tool from a MP600 will just drop right into a MP400, no modifications required. Hope that helps.
