I’ve just become aware that there is a manuscript in the holdings of the Bodleian Library, catalogued as a “Syriac” manuscript (MS. Syr. f. 2), which is in fact in Mandaic. If you click through, you can see some images that the Bodleian has shared. Click through to see the images.
There are notes on the scroll in French which may suggest that the text relates to the Mandaean holiday of Paruanaiia, celebrated during the 5 intercalary days that allow the Mandaean calendar to have months of even length (30 days) but an essentially solar calendar of 365 days.
I haven’t (yet) tried to see if I can figure out, on the basis of the small number of images provided, whether the text is identical to another known or published work. Based on the description, it might prove to be another copy of the work which is in the Drower Collection (DC 24), Sharh d-Parwanaiia, “The Ritual of the Five Days Intercalary Feast”, which was recently translated into German by Bogdan Burtea.