unknown MGBs

It's all a bit puzzling isn't it, if your German source is taking his info from wartime compiled records which I know personally are not that efficient and you seem to have the answers to your own enquiry as it appears there was no such boats in the European Theatre, however it is quite possible that a boat, any boat with guns on would be identified as an M.G.B as American P.T's were a copy off a British design. The same as the British used the general term "E" boats for German gunboats. The other thing that is noticeable is that when you look at M.G.B. R.N. numbering there is nothing between 192 and 312 (Fairmile C) leaving all of the 200  number's vacant I wonder why that was?, could it have been for the American manned launch number's in European waters? surely it would have been easier to change  say P.T. 206 to M.G.B. 206 rather than give it a complete new number on transfer to the R.N. The only other group of boats with a 200 onwards number are unarmed R.A.F. 371/2 ft S.T.'s, as to whether the Germans shot any of them up Donald would be the best person to ask. Knowing how good the American's are at looking after crew comfort were the not Pacific spec boats different to the one's used in Europe?

As you appear to have rubbished all other suggestions I have put forward in trying to help you with this puzzle in regards to the identity of the mystery "M.G.B's" and as your info source comes from records perhaps the definition M.G.B. does not mean MOTOR GUN BOATS at all but MOTORISED GUN BOATS these Thornycroft designed 105ft  14knt wooden hulled vessels  a.k.a. LCS(L) (2)  have a similiar profile to an M.G.B. but with a flattened prow they were a well armed vessel and the numbers you quote fall within this group, unless of course you know better, otherwise this subject is about exhausted I think don't you.

dldldl, perhaps this could be worked at from another angle.

Instead of trying to figure out what the boat may have been from your sources. If you gave some information as to what the context of the reference was, e.g. what engagement where and what the outcome was it may be possible for some of the clever gents on this site to identify the vessel from this end.

For example what vessels were operating in that location according to our records?

LCS (L) (2) Losses 1944 ; 252, 256, 258