That's a possibility Andy, another good suggestion, could well be. Conditions a bit good for that though and the barrel and receiver both have the same early factory finish on. It would seem odd too if that were the case that it would not be clearly marked "Factory Use Only" or a variation of that.
The thing is Chris, I believe (tell me if I'm wrong) that all the Enfield marked examples were only assembled and marked En at ROF Enfield from components supplied by HK as opposed to being manufactured there. I think this was done to circumvent the Arms export restrictions that Hk were working under back in the late 1960's and early 1970's.
That's why my initial thoughts on this piece were (probably still are) as a Hk supplied early manufacture MP5, as a parts kit, or as a complete firearm, to help with setting up production at ROF Enfield. 1968 is about the right time frame.
Perhaps it was supposed to be assembled and numbered at Enfield and for some reason this was never done ...
I have a variation on this theory, Perhaps it was supplied to, but never assembled at Enfield and was only completed by the RFD who bought it from the Enfield site as it was drawing down toward closure.
We would need input from a former Enfield employee who worked on the MP5 project to shed light on this. I do know an awful lot of Firearm parts went into the trade when this formally great British institution closed.
I have contacted the Enfield Apprentices Association before, in an effort to gather information on MP5 production, but never received any response unfortunately.