Hi Mark,
It's odd, your first set of photos (from Jun 24, 2012->Mar 21, 2014) do not prompt for a login to google. But these newer links do.

You can test by opening your link in an 'incognito', 'inPrivate' or 'private' browser window...this strips out the possibility you might be signed into your own account.

But it is curious; I too can see that your newer albums are 'shared publicly'. I'm not sure why the newer links prompt for login.

But check this out...if you make the URL resemble that of your original album, I believe that'll load without requiring a google login. (https://plus.google.com/photos/<your_google_uid_number>/albums/<album_uid_number>)

For example:
power distro panel:
https://plus.google.com/photos/115050180406101428512/albums/5997804018197848193

radiator album:
https://plus.google.com/photos/115050180406101428512/albums/5999850918500586641

Thanks for sharing the progress!

1958 Chevrolet Viking 60
Assortment of photos