The M48BO rifles were produced for a military contract that fell through. They were never sold, nor issued, and sat in the crates for 50+ years. Marstar had these six or eight years ago, and they were indeed "unfired and un-issued".
My experiences with the BO rifles differs from yours substantially. For example, there were multiple production runs of the BO rifles, this is pretty clear. Roughly half are M48's and the other half are M48B's. No M48A BO rifles has yet surfaced that were not obvious refurbs.
Milled M48 rifles were only made from 1950 to 1952, this includes the BO versions. Only 238,515 M48 variant rifles were made. This is an established fact from the Zastava factory records.
M48A production ran from 1953 through 1956, during which time it appears no BO rifles were made. The M48B rifles, including the second batch of BO variants, were made from 1956 onward.
Many BO rifles were imported to Canada and the US in obviously used condition and many examples also show evidence of being re-worked. It's also possible (and even likely) that BO rifles have been assembled with recycled parts from Yugoslavian military stocks, but with markings scrubbed.
I have personally owned a number of M48 series rifles, including BO variants, where there are obvious pre-1948 proof stamps (T supplanted by either a crown or star) on some parts, such as the bolt body. This could only occur if parts were cannibalized off earlier worn out rifles and re-used in the refurbishment process or during production. MY current M48 rifle from Tradex has a re-manufactured bolt on it that would originally have been on an M1924 carbine. The bolt has a more serpentine bend to the handle than a typical M48 bolt and the proof stamp is a T with a star over it, not a BK in a circle like a typical post-48 bolt. All the parts on the bolt body itself are later production parts.
Brando Bogdanovic (author of "Serbian and Yugoslav Mauser Rifles") and I used to correspond about this back when he had just released his book. It was Branko's opinion that many of the rifles being sold in north america had been very recently referred to facilitate the sales to American surplus dealers. It's been years since we've spoken, but I trusted his opinion back than, at the time he was the official historian for the zastava factory.
Here's an interesting photo for you:
Notice the assembly numbers on this M24/47 rifle. All M48 variants also have this assembly number present. The very great majority of M48's in Canada have bolts with mis-matched assembly numbers, despite having a matching serial number. This is effectively proof that most of these rifles are rebuilt. If you check M48BO's, many of them are similarly mis-matched for assembly numbers.
I pulled the hand guards off two of the 6 current batch trades guns I had access to, and on both, the assembly number for the bolt did NOT match the barrelled action, so both were refurbs, despite no outward appearance of overhaul. The gun I kept also has a non-matching assembly number.
It would be fun to see more owners post their assembly number results as they clean their guns.
Another interesting bit of data. Next to the assembly numbers, there is often one or more punch marks. This is popularly thought to be applied when a rifle was re-barelled during refurbishment. On all the guns I've owned with pristine bores, these punch marks were present, and sometimes more than one punch mark is found denoting multiple overhauls, at least two of which involved new barrels. When the shooter grade guns were sold in the USA in the early 2000's, I seem to recall many had matching assembly numbers and less nice bores - and few had punch marks.
