It is japanese standard of standard since very beginning of industry standard in Japan ( at the start of weapon production in Japan at 19th age ).
The same part number could be assigned to different units. Speaking about Okuma you need to provide machine and OSP serial number to get the order of spare parts correctly.
Speaking about SDU, the part number can differ despite the module is exactly the same :-) . Maybe the unit is modified. Must be interchangeable.