Unfortunately I do not have the 4242 and 423 second edition, so I cannot compare them with the ones in the 29660 starter set.
From photos they seem to be rather identical, except for the number.
4242-1, Relex couplers, no. 21402, bogies type 11-a, bellows 3 mm long
3242-2, close couplers, no. 21402, , bogies type 11-b, Bellows 6 mm long
4243-1 , Relex couplers, no. 19926, bogies type 11-a, bellows 3 mm long
3243-2, close couplers, no. 19926, , bogies type 11-b, bellows 6 mm long
29660-2

close couplers, no. 19923, bogies type ??, bellows 7 mm long (measured by me, below the end window)
29660-3

close couplers, no. 19925, bogies type ??, bellows 7 mm long (measured by me, below the end window)
To compare:
4243-1, Relex couplers, no. 19926, bogies type 11-a, bellows 3 mm long

I have installed close couplers Märklin 7205 and a long bellow in one end
29660-4

close couplers, no. 21401, bogies type ??, bellows 7 mm long (measured by me, below the end window)
4242-1, Relex couplers, no. 21402, bogies type 11-a, bellows 3 mm long

have installed close couplers Märklin 7205 and a long bellow in one end
Please note that the NSB F1 model from Märklin has different window arrangement and sign text on the two sides.
Edited by user 04 December 2013 22:44:39(UTC)
| Reason: Added more information