It’s somewhat humorous to see incorrect word forms. Some mistakes are from mispronunciation, such as ax for ask or supposably for supposedly. Others are based on a misunderstanding of the word. In the field of engineering, one of the most commonly mistaken word-forms is the plural for Bill of Material.
Of course, the term is Bill of Material or Bill of Materials for singular form. In this case, the words material or materials both refer to the total sum of material that makes up the assembly; they mean the same thing (kind of like flammable or inflammable). The plural for Bill of Material is Bills of Material, as in multiple bills, not multiple materials. This is similar to Flights of Fancy, Peas in a Pod, Chariots of Fire, and Dogs of War. I guess some confusion comes from the abbreviation for multiple Bills of Material, which is commonly BOMs. It would rather silly to try to pronounce BsOM.