The VW Beetle has spent the entire fifties in the lead and still held a huge 18.6% share in 1960. It led the market again in 1961 and 1962 (market shares unknown – please get in touch if you have info) but is down to #2 in 1965. The Volvo PV 444/544 was #2 in 1960 with 14.7% share and stopped being produced in 1966, therefore being the least likely candidate for the pole position over the period. However the Volvo Amazon, only 4th at 8.1% in 1960 is the best-seller in 1965 and could have spent a couple of year on top before then.
If you do have sales data for Sweden over the period please ensure you get in touch by commenting on this article.