Hi, I finally managed to restore the serial number using Maui Meta. In practice, the barcode field (used in Maui Meta) have 63 or 64 digits/characters: the first 23 digits/characters refer to the product number, the other ones refer to the serial number. I had to fill all 63 digits/characters, using spaces at the end of product and serial numbers. I'll make an example (look at the quotes, the underscores represent spaces, I used underscores here in the post because spaces were automatically trimmed):
Product number: "ABCDEFGHI"
Serial number: "0123456789"
Barcode string to use: "ABCDEFGHI______________0123456789______________________________"
The product and serial numbers finally appeared in "settings -> about phone -> status" menu. The original suggestion is for Lenovo P70-A but it worked with my Lenovo A2016a40, so I think it could work with other models too.