French 81mm mortar,which fuze?


I picked this French 81mm mortar up a while ago,it has been described to me as a training round.
Does anyone know what type of fuze would have been used in this mortar please?

The usual type of fuze won’t fit,as the mortar appears to have a different fuze pocket to those seen on other types,on my example it has straight edges,while on other mortars the fuze pocket is curved.(Hope this makes sense?!)
Also,the size of the opening in my fuze pocket is huge compared with ‘normal’ 81mm mortars.
Is there some kind of adaptor missing from mine which enables the fuze to be screwed in?

Many thanks,