The CAA AAN seems to shed some light on the stretched Gazelle.
It appears that there are actually two versions of the stretched Gaz!!!!!!
Firstly there is an Aerospatiale mod 341.MR.0480. This is acceptable to the CAA.
Secondly there is the American stc which is said to be "similar" to the french mod. However to be acceptable to the CAA this has to be shown to be in compliance with the French mod.
How many came off the production line like this and how many were modified in the field I do not know.