An advertisement I've found appears to backdate the release of the Model 1 by a couple of months:
June 8th, 1955 - Richmond and Windsor Gazette
I've also tracked down Merv Richardson's earliest patent, which predates the release of the Model 1 by only a couple of months. It possibly also represents one of the earliest illustrations of the mower:
Last edited by CyberJack; 30/12/1507:52 AM. Reason: Added attachment.