The hand is quite rough and inconsistent - not very beautiful. There is a great deal of variability between letters. I'm not sure whether to criticise the wavey lines as they may have been induced by the medium - birchbark does this I think. Reading through the text there are a number of corruptions and omissions. But we should not be surprised by this as it is common. And given that the scribe was a sloppy writer it is not a stretch to think they were a sloppy copyist as well. There is what looks like an explanation of the mantra (perhaps pronunciation instructions?) in another script which I cannot identify at the end.
One of the features of the writing is that the pen is held close to vertical. This gives the syllables a 'blocky' feel with heavy horizontal lines. Another distinctive feature is that words are separated by a dot - this is not quite like the Tibetan tsheg which separates syllables, but not an Indian practice either.
The akṣara heads are simply a horizontal line which is typical of manuscript Siddhaṃ - the elaborate wavy lines on Japanese Siddhaṃ appear to be an Asian innovation.
A long ā diacritic moves diagonally up and and away from the head, rather than either vertically or curving downwards. (see image right)
If you are used to contemporary Asian Siddhaṃ then some of the letter forms might appear unusual. I've cut out some of the interesting akṣara and labelled them. The ca is similar to Tibetan forms (which are more closely related to their progenitors than either Siddhaṃ or Devanāgarī). The ṇa is like nothing I've seen before and I only know it from the context. Ta (here with the ā diacritic) is more like a modern bha, whereas the bha is closer to the Lantsa bha (again I think this is an earlier form preserved). Note that sa and śa are very similar. In fact sa looks like a Devanāgarī bha and śa like a Devanāgarī sa.
I think this text demonstrates how difficult it can be to move from the clean lines and tidy arrangement of contemporary Siddhaṃ calligraphy to the manuscript tradition. I can more or less read this text as long as I have a Sanskrit Heart Sūtra handy to fill in the gaps. But I have seen a lot worse than this!
If anyone has more information about this manuscript - such as where and when it is from then I would love to hear from you.
Note 29.11.12. I have reason to believe that this ms. is the one referred to by Edward Conze in the notes to his critical edition of the Heart Sutra: Conze (1948: 49), and Conze (1967: 154) Ms. Nm/Cg.
Bibliothèque Nationale [de France] 62 no.139. Pelliot Sogdien.The text is the same as that published in Benveniste, Émile. Textes sogdiens. 1940; the image file name includes the text "Pel.sogd". I can't find the image or the record for it on the BNF site, so I've written to them for confirmation, but I'm fairly certain.
The ms. dates from ca. 1800, and is written in the Sogdiana form of Siddhaṃ. It was found in Dunhuang by French Sinologist Paul Pelliot in 1910.</