bellmac – Outline scheme for form letter production
The scheme is part of the author's AutoLetter package.
Maybe you are interested in the following packages as well.
- knuth-letter: Knuth's example letter macros
- lettermac: Simple macros for writing letters (Plain TeX)
- automagic: Automagic numbering in Plain TeX
- twoside: Two-side typesetting in Plain TeX