Where can I find the part number or serial number on my power device?

First, take a look on the underside of the charging unit, meaning the part that normally makes contact with the top of your desk / table. Normally, there is a sticker somewhere, which contains a part number and usually the product name next to it. 4Pxxxx is the most common structure but it may start with something else. Significantly older models may instead start with "KHX-" or "HX-" alongside a number starting with "3500xx".

If you see an indent in the charging unit that looks like it was supposed to hold a sticker, but do not see the sticker, the sticker may have fallen off.