I just replaced my mast and opened the top, middle and bottom of the old mast without evidence of any foam or other floatation items noted. Someone had posted they thought the 1980's DS 3 had something in the top most portion of the mast. Not in mine.
As a side note, I bought insurance for $150/yr, filed a claim 3 months later for a new mast plus all rigging. Insurance did not bat an eye, paid for the whole thing plus a few extras. Only got stuck for a small deductible, they paid $2,200. Did pretty good at the metal recycle center for the Al.
Please do not asked, I did a very dumb thing and lost my mast.