Hi Steve,
Here's my take on the tunic and the possible regiment it's owner was assigned too.
Firstly there is no providence with this item so it's purely conjecture on my behalf.
The tunic came to me as is I have not added or taken away anything, it is a far as I can deduce a privately made tunic in the post 1915 style and fits all the stipulated "size and style" requirements of the uniform regulations with the exception of the top pockets. This feature with the lack of unit acceptance stamps leads to the assumption it's for an Officer.
The Officer Boards are Prussian flecked and are the correct pattern for the tunic as a late war garment, they even measure 4.5 cm exactly as per regs. When you look at the stitching on the shoulders does not indicate that the boards have been added since the tunic was made and when you move the button the fading of the board compared to the colour under the button indicates they have been on there long enough.
The number 8 is a sticking point.. as you rightly say the Grenadier RGT would have the cypher monogram not a number, also why brass and not subdued number ? There has only been one number or attachment to them and no other pin holes are present. Don't think its a reserve unit but not got enough facts to discount it yet. The other thing is MGK were prevalent in a lot of infantry units at this late stage so it can't be an abteilung number in my opinion.
The question is then, where the numbers just added later to spice up the tunic ? this always a possibility as the indent and fading are not as pronounced as the button ones.
Second theory is it may have been put to use post war by the owner as part of the new Army. Again only conjecture but the condition of the tunic implies that the owner only used it lightly, maybe he was KIA or it was a second tunic ? If he was moved to the Provisional Reichswher post 1919 then RGt8 was a Prussian unit ( interestingly Komp1 was formed from Grenadier RGt 8 of the old Army) and the MG qual badge may well have been worn if he was in the MGK kompany as I understand Officers may have also been trained with the other ranks in using the heavy weapons so why not wear the badge.
I will attach further pictures below of the tunic and welcome other opinions and thoughts positive or negative either way.
cheers Tony