alternate product name for invoice pdf only

[herr.proof]

Hullo!
A client of mine asked for an alternate product name to be printed in invoices (pdfs). 

e.g. : product id 51, product name "myproduct51". on site, all goes well.

in invoices  'myproduct51' should be something else, say "Alternate-name-for-item-51" 

What could I do?
Any hint will be greatly appreciated.
Thank you in advance!!