Hi BlackMilk, Unfortunately, there is no clear date for the change from the solid engine deck to the twin rail, but I can tell you that in the Selected Product Range in 1974 - 1975 the Models 450407 14"; 450423 17" and the 450431 20" all had the solid engine deck fitted.
The 14" had a 2.25HP Briggs and the 17" and 20" had the Kirby Tecumseh/Lauson engine HK30 3HP engines fitted. However, some 17" machines may have had the HK25 2.5HP engine fitted.
By 1978, Scott Bonnar had changed to the Briggs and Stratton engine for the
entire range and also
all Models of the 45 were by now fitted with the twin rail deck.....
See the brochure; Click
HEREThere have been cases of fatigue cracking of one or both of the twin rails....but these are easily repaired. Choice is up to the individual....the solid deck may be better, but there are output shaft size differences between the Kirby and the Briggs.....so extra expense there fitting the later Briggs to the early chassis.
