Thanks guys for yours comments about my stuff

.
To Dog Smack :
I use very thin iron wire and brass wire a little thicker.
The iron wire is used for the ring on the trailer while the brass wire is used to make the hook on the tractor (brass wire is harder than thin iron wire).
Few examples (photos taken quickly

) :
first example, hooks on two US M3 halfrack (work in progress...):
on the second picture you can see better the iron wire glue in front of the trailer to replace the original GHQ ring part.
The same principe on this sdkfz 10 and Sig 15cm : the brass wire is glue (with cyanocrylate) at the rear of the vehicule (or undercarriage) and the thin iron brass is glue (with cyanocrylate too) on the trailer (or gun).
Sometime, I preserve the original pewter ring : I make a very small hole throught the ring with a very small drill.
See the example on this german Flak38 trailer :
...and it works !
Could be great on sdkfz 251 C & D
On the "ausf. C" with the 12cm Mortar, the brass wire was glue in a hole* (*I make it with a small drill) at the rear of the vehicule.
On the "ausf. D" (with the Pak 40) the brass wire was glue under the vehicule with a generous coat of cyanocrylate.
Look great ! and it's very easy to remove the trailer or hitch it. (and very easy to move the vehicule on a wargaming landscape : take the tractor and let the trailer slip behind gently on the ground)
Look great with Mac NO too :
You can see the very small hole on my Luftwaffe trailer to hitch a brass wire hook.
I hope it could help you.

Pibber.
(Long life to GHQ !)