Siddham did not stay a handwritten script. Once printing from carved wooden blocks became ordinary in East Asia, Sanskrit syllables were cut into blocks along with everything else, and printed dharani sheets, charms, amulets and canonical texts carrying Siddham were produced in quantity. A block carries the letters as an image, so it is indifferent to whether the person operating the press can read them, and that indifference is exactly what allowed the script to be multiplied by people who could not.
Printing changes a script in two directions at once. It fixes a form, because the block preserves whatever the cutter cut and every impression is identical, and it also degrades the form, because a cutter working from a manuscript in a script he does not read will simplify what he cannot interpret. Both are visible in printed Siddham: some editions are careful and follow the manuscript tradition closely, and others reproduce letter shapes that have plainly been copied as pictures.
The result is a corpus in which the same syllable can be found in several conditions, and a reader has to know which kind of witness is in front of them. The distinction has a practical edge for anybody working from reproductions today, since a syllable copied from a late printed charm and a syllable copied from a manuscript exemplar are not equally good evidence of what the letter is.