By explicitly associating Mozart’s portrait with his printed music, and the manuscript of K20 with a ‘sounding’ piece of music – the British Museum’s description of the sonatas and manuscript as ‘musical performances’ is significant in this respect – Leopold Mozart effectively makes both the physical objects, the music print and autograph, and the immaterial object itself, the music, souvenirs. To fully understand this notion in an eighteenth-century context, it is necessary to consider the meanings of the word 'souvenir' at the time.
Prior to the nineteenth century, the word souvenir – now generally understood as 'something that is given or kept as a reminder of a place, person, event, etc.; a memento, a keepsake; spec. a (typically small and inexpensive) item designed for sale to tourists and having some association with the place visited’ (OED) — was only infrequently used to refer to a material object, and only towards the end of the century. Derived from the French souvenir, it represented memory associated with feeling, whether that memory was buried deep in the unconscious and had to be willed into consciousness, or whether it was on the surface of consciousness, ever-present.
In eighteenth-century German writings, the word souvenir generally appears only in passages that, unlike the rest of the text, are entirely in French, or it appears in a font (italic or roman, depending on the original language) that distinguishes it as French in both origin and meaning. According to Grimm, before about 1800, 'souvenir' is rendered in German as 'andenken', an evocation of thought and memory. It is only toward the end of the century that 'souvenir', more explicitly rendered as 'äuszeres zeichen' and, later, as 'geschenk zur erinnerung', becomes common as also representing an object.
Similarly, in English-language texts, including both dictionaries and literary texts, the word souvenir generally occurs only when a passage — a quote or a letter — is given in its original French and is used to mean ‘memory’. The earliest usage of the word souvenir to refer to an object listed in OED is the 1776 stock catalogue of the jeweler J. Frankland: ’A souvenir mounted in gold.'
All of these uses of the word souvenir take as their basic meaning the French souvenir, representing memory, a multifaceted concept defined in Diderot and D’Alembert’s Encyclopédie as represented by four related words: mémoire, souvenir, ressouvenir and réminiscence:
- These four words express equally the repeated attention of the mind to ideas it has already perceived. But the different nuances attached to this general idea … gives these words distinctive meanings. . . Mémoire and souvenir express the attention of the mind to ideas it does not forget, although those ideas no longer occupy it: the ideas have made permanent impressions; one casts a glance at them by choice, it is an action of the will. . . Mémoire however is concerned only with the ideas of the mind; it is the action of a faculty subordinated to intelligence, it seeks to enlighten. Souvenir, on the other hand, is concerned with ideas that interest the heart; it is the action of a faculty necessary to the sensibility of the soul, it seeks to arouse
 See OED: Etymology: < French souvenir act of remembering, a memory (end of the 13th cent. in Old French), something that serves as a reminder, memento (1676), narration or account of memories (a1729 as a work title), use as noun of souvenir to have (something) in one's memory (c1100 in Old French as suvenir ), to recall (c1130–40), to commit to memory (1176–81), to remind (someone of something) (c1265)
 The text of the Encyclopédie is available online at http://diderot.alembert.free.fr.