Up to 1974 all Spectras (18, 20 & 24's) had 4 full stringers with center bulkhead and the deck had a foam core. They were layed up very heavy!
From 1975-1978 the jets (18, 19 & 20's) had motor boxes.
From 1979-1981 Spectra went back to full stringers on the jets & I/O's.
Spectra always had full stringers & center bulkhead in the v-drives (19, 20 & 24's).