I think that is what Bruce wrote in his bayonet book, something along those lines, a naval ordnance shop. Some of the G98's look to be made up of leftover receivers, no signs of previous use, and only the M/A for a FP, so it probably does represent an ordnance facility build in those cases, but not enough examples, especially in original condition, or well documented, to know what the deal is for sure. At least as far as I have recorded them.
One thing is for sure, German naval related rifles are a queer bunch, more variation than the army examples, which seems to hold true in the Imperial through the nazi era, there are several odd variations or markings seen, and MauserBill once told me that his experience has shown them to be sloppiest of the bunch, probably due to the nature of the service, - the last to get small arms and having to do a lot of "make do" with salvaging. It is well known, or written about, that when WWI started they got their modern rifles plundered by the army, to the extent they could spare them. Seems the same in WWII, if not plundered they had a hard time getting new rifles, as evidenced by SDP relationship with them, most G29/40's went to them, most of the builds associated to the KM were Radom G29 parts, or G29/40 parts.
Maybe the OP can do a picture of the barrel, removal of the handguard should reveal if the barrel is a replacement, also any markings on the stock, if any exist, the pictures i have do not show markings well, looks like a cypher and acceptance might be on the RS, but very faint.
Bruce's page 100, I doubt he would mind, for those that do not have the book, which is hard to find today.