Freebsd – How to apply updates on OpenBSD, NetBSD, and FreeBSD

does that mean that OpenBSD released the very first batch of source code in "19 May 1998"

No, the first public release of OpenBSD was made in July 1996. (Source 1)

19 May 1998 means your paper's authors used OpenBSD 2.3 as their starting point. (Source 2) I don't see any special reason to begin with this release. The explanation is probably something trivial; perhaps that was the oldest version they could readily lay hands on.

where did the other 61% of the code base come from,

NetBSD. (Same sources as above.)

was that never posted to an open source version control system?

The OpenBSD CVS repository appears to have been in continuous operation for the ~15 years of the project. I fully expect that you could check out OpenBSD 1.x from there if you wanted.

where would I find the source code for the patches in those 7.5 years,

The CVS repository.

do those dates match releases dates,

See Source 2.

I'm unable to tell if that release was open source.

OpenBSD has always been Open Source. It was a fork of NetBSD, which was itself Open Source.

The reason why you cannot just mount the partitions is because you have a disc image not images of individual partitions. You would need the offsets of the different partitions and use those when mount using its the loop and offset options.

In your case I would play back the image and then upgrade, but you don't indicate how you connected the 2.5" to you desktop computer, or how you are going to do that now. If you are going to use USB, then upgrading before playback is probably faster (but a bit more work), assuming the image is on an internal SATA drive. Because of the USB 2.0 vs SATA speed differences the upgrading is going to take longer. The playback of the image (upgraded or not) will take the same time.

If you want to upgrade before playback, then use parted to find the start of the images. parted hd.img will give you a list of partition numbers, start and end. With the start information e.g 12345 you can mount a partition in the disk image:

mount -o loop,ro,offset=12345 hd.img /mnt/tmp

You might need to specify the partition type as well if your desktop does not recognise it. You can then update your fresh install with that info, remount the partition rw clean out the partition and write things back. The only thing I am not sure about is if that would consfuse the ATX board's bootloader, but upgrading a system restored to disk would have the same problem.