As of today (26 April 2015) there is a set of MG34 Lafette "Inner-Rails" for "auction" on E-Bay-DE (Germany).
(Not my auction).
I know of at least two (2) MG42-Board members that are looking for these parts.........so I would recommend that you talk to each other prior to getting into a "bidding-war".......(?).
Yes, they are rusty......."but" you may be able to cut the required mounting-brackets off of this and weld then onto the inner-rails from an MG42-Yugo-Lafette (?)......or use them for measurements to reproduce these for others on the MG42-Board (?).
Note: Please don't cut up a WWII MG42 set of Lafette inner-rails as I am in need of a set of these.
US Army, Retired