The oldest pattern that I found was number 824, which also appeared in the earliest Poppleton's ad that I have seen, in Vogue Knitting book in 1949.
When I am sorting patterns, I look out for the ones that we have the largest number of copies of - I assume that those patterns are the ones that sold best, and so are the ones that were most attractive to knitters at the time. One popular Poppleton's pattern is a picture sweater, with a country cottage on the front - the leaflet was advertised in 1981.
Poppleton may seem like a jokey sort of name, but it comes from a village near York (or in fact two villages, Upper and Nether Poppleton). And anyone who reads The Times Higher Educational Supplement (or has read it at any time over the past 30 years) will be familiar with the University of Poppleton: Laurie Taylor writes a regular column, The Poppletonian, about events there.