There has not been much research, nor even speculation, about this card. The most recent author to discuss it is Giordano Berti, in his "Storia dei Tarocchi" (2007):
The Sun is represented differently than normal, that is, above three hills on a black and white checkerboard pavement. These mountains are probably an emblem of the Christian faith, given for a reminder of the three crosses of Golgotha. On these latter it is possible to read the letters MAC, for which no explanation has yet been offered. We can suggest a possible reference to the principles of the beatitudes given by Jesus to his disciples in the Sermon on the Mount, that is to say M(ites) (meekness), A(gritudo) (sic. should be "aegritudo"(?) affliction), C(haritats), or C(astitas),...
Berti draws attention to two remarkable features of the card - the objects that look like mountains, and the letters on the mountains. But there is a more obvious interpretation than his rather fantastic analysis, which is that the card represents a heraldic device. Both Michael Dummett and Detlef Hoffmann made this observation:
Another [card] shows a sun, with rays and a face, very like that of the Guildhall Ace of Swords, above a chequered floor on which stand three metallic objects bearing respectively the letters a, m, c (perhaps heraldically conventionalised mountains, or perhaps something quite different).
The Game of Tarot (London, 1980), p. 73.
As for the sun above three stylized hills, this might be able to identify it as a Tarot (but perhaps it is also an example of heraldry.)
D. Hoffmann, in T. Depaulis, ed. Tarot: jeu et magie (Paris, 1984, cat. number 6, pp. 39-40)
Beyond these offhand suggestions, nobody to my knowledge has seriously looked for heraldic insignia corresponding to the card. So, when I first visited Rome and Siena, I was happy to see some heraldry that at least resembled the card.
From the Piccolomini library, Duomo of Siena Despite the poor quality of my photograph, the name Savini can be seen, and the mountains are definitely the same stylized mountains of the card. This Savini appears to have been Choirmaster at the Duomo in the mid-15th century, and looking on the web for further images of the arms brings up a few, like this one.
It's obvious, however, that this is not identical to the Goldschmidt Sun card - first of all, there's no sun - the shining object is a star, not the Sun. Secondly, there's no cross, and lastly, there are no letters on the mountains.
Continuing the search with terms like "tre monti", I found this one from the Bertoldi or Bertoldo family, prominent in Venice and Padua. The description of the arms reads "Arma: d’azzurro a tre monti accostati di verde; ad un sole nel punto dal capo figurato d’oro."
(on blue with three verdant mountains; a sun at the top point in gold)
From: the "Sustinenza" webpage.
Finally, there was a Sun - but still no letters or cross. Nevertheless, it began to look like heraldry would provide the answer. In the meantime, Michael Hurst also found a device from the family of Sonnenberg (appropriately), which had three mountains surmounted by a blazing sun.
We seemed to be getting warm. I also managed to find two shields that had three mountains surmounted by a cross.
However, at the end of the day, neither of us could find one that matched exactly the Goldschmidt card. But the hunch of a heraldically inspired design seems to be the best way to interpret it, rather than obscure allegory.
The stylized mountains are common in heraldry. One very prominent example is the Chigi-Saracini family's arms, in Siena, with six mountains surmounted by a shining star.
In this case, a motto is given as well - "Micat in vertice", which means "It shines at the summit". This example made me wonder whether the letters on the Goldschmidt card could be an abbreviation for a motto.
The letters can be read "a.m.c." or "m.a.c.", and so far I have not been able to find a corresponding Latin motto that convinces me. One possibility is the Latin term "ad medium caeli" (or "celi" or "coeli"), which is an astronomical-astrological term meaning "at the center of the heavens". It is abbreviated MC in astrological charts. It seems appropriate for a blazing - noonday like - sun, and that the letter "m" for "medium" would be on the middle hill, but so far I have not found anybody who has taken it as their motto.