Bill Cawthon has written a great article about the history of The Saint's Volvo that features a couple of nice pictures of early models.
One of the interesting things that is discussed in the article is that after Leslie Charteris sold the television rights to Producer Robert Baker and Lew Grade of ITC their first visualization of Simon Templar on TV was of Patrick McGoohan driving a Jaguar E-Type. When the producers went to Jaguar they were turned down as Jaguar felt they had enough exposure and could not keep up with the demand as it was. Therefore they ended up with the other hot car introduced at the Geneva Auto Show in March 1961 by buying one at full price from a London Volvo dealer. Luckily, as the show took off , Volvo kept the show well stocked with fresh 1800's as the years went buy. Older cars were cut apart for interior shots, crashed, and otherwise used by Roger Moore as his personal vehicle of choice.
YouTube also have an interesting five-minute video about The Saint' Volvo 1800.