My personal opinion is that it is a seedling of Peace.

https://www.helpmefind.com/rose/l.php?l=21.122337

You can see Peace-like new growth in that photo, but also S.Silver has broader petals and decent form, which was rather difficult to get from most lineages at that time without using Peace. Also, Peace provides color disambiguation that silvery mauve often requires to be present.

The other half is obviously the mystery meat. I think its from an early pelargonidin types for several reasons. First, its a shorter rose than most HT. This happened in many early HT (Ophelia types, for example), but not as commonly during the 50s. Although this can be partially blamed on weak vigor due to selection bias of a rare color, it seems unlikely in this case, because the internode relativity seems to contradict such an idea. So it was probably both short and weak, rather than simply weak. I believed a dwarf variety is the culprit, such as those found in many early floribundas carrying pelargonidin. Second, the timing is perfect for the when such unusual silvery mauve colors were truly possible from these early pelargonidin types. Orangeade, as an example, has an extensive history with silvery mauves. 'Lavender Delight' is a prime example. Orange Sweetheart, for example, was introduced during the timeline when Morning Mist (a possible relation or sister) was being conceived. Last, to me it looks like the parentage of Morning Mist was deliberately concealed to hide the rose responsible for the color shift, only offering the idea that it was bred from roses that could typically make a florist rose at the time. Yet, the petal count and flower size is more like that of an early flori or flori hybrid.

If I had to guess, I would guess 'Fashion' was involved. I would guess a red florist rose derived from the roses she had listed and Fashion, and that the result was an odd-toned darker mauve.

I would also hazard a guess that 'Morning Mist' is a self of this unknown seedling, and that when she realized the outdoor vigor was even worse than the florist rose it was bred from, she crossed it with 'Peace' and got a silvery, better formed rendition.

Well, anyway, thats my best guess with how much little information is available.