Here is a photo of the Ludwig replacement for the Marlin cross-bolt safety. You can see both sides from the picture. IT IS NOT A SAFETY!!
It just fills the hole where the safety would go. Looks like a screw on one side and a pin on the other. Took me 5 minutes from removing the stock to replacing it after doing the job. p.s. have two Marlins so that's why you get to see both sides in the same picture.
Here is the URL for the instructions:
http://www.sixgunner.com/comments/safetykit.htm Kits are available from:
Clyde Ludwig
PO Box 26156
Wauwatosa, WI
53226-0156
phone 414-536-1101 after 6:30 PM CST
$9.95 plus $3.00 S&H
(if you order more than one kit, the shipping is still only $3.00