I've downloaded the entire archive of CRAN (don't ask we why yet) and I see some old versions of packages do not have a NAMESPACE file, but I'm pretty sure they did have exported functions. Was there a different mechanism in R 1.5?
https://cran.r-project.org/src/contrib/Archive/zoo/zoo_0.1-1.tar.gz (last modified 2004)
├── COPYING ├── COPYRIGHTS ├── DESCRIPTION ├── INDEX ├── R │ └── zoo.R └── man ├── as.zoo.Rd ├── index.Rd ├── plot.zoo.Rd └── zoo.Rd
I'm very curious!