"Appendix. Proofs and illustrations": p.[29]-52. Three of the letters included are dated Dec. 26, 1798. This and the date Dec. 24, 1798, when the main work was ordered to be printed (cf. p.27) suggest that the complete work may not actually have been issued until early in 1799.
Microfilm series: Early American medical imprints, 1668-1820, reel 27, no. 497.