I have tried to figure out what this rose was, or at least an estimation of what it was.

I know Lammerts, as minimum, had an informal relationship with G. Buck. This rose doesn't seem like it would fit in the timeline of Lammerts more formal, commercial relationships. So I imagine it was either done by himself or in conjunction with G. Buck. It's possible that this is either a seedling Buck may have used in his hybrids, gifted by Lammerts, or something they worked on in idea together, and was introduced by Buck. An example: Red Dream.

Of course, the likelihood of any of us ever knowing is quite slim, but that is as far as I could hypothesize as of today.

Another hypothesis could be the maternal grandfather of Carefree Beauty.

Edit: Another hypothesis would be 'Brave Patriot'. We don't know when it was a found mutation. Only when it was introduced. Also, it is claimed to have high mildew tolerance.