Guys you are Giant addicted!
Tommy your two SS are fraternal twins, very similar, but there are few differences between 87 an 88 models.
Can you check the seatpost diameter, please? I have a doubt.
Thank you Tommy, I raplaced the old one with a SR that I had fortunatly at home. Unfortunatly it's an earlier model but it fit perfectly. It's marked 26 but decimal point could be ruined.