Utter rubbish, as said SB's are NONE mandatory, if they were Mandatory you would see every Cessna 150 152 with about 10 fuel drains per wing....... never seen one with them fitted.
There was a CAA letter that came out a few years back trying to clarify the position on SB's but as much as I have looked I cannot lay my hands on it.
Really you want to be asking this in the engineering forum, a lot of us would point you in the right direction.
There is a company in Germany rewebs/overhauls seatbelts and they come with an EASA form 1 for what its worth.