HVA 400 series vs 500 series
One nut is probably OK for the 400-series of plastic farmers saw and the torsion forces involved from the moderately sized bars used.
However, if using longer bars, go for the more rigid magnesium body of the professional saw series, such as 545, 550XPG, 555, 560XPG or 562XPG.
P.S.
I can tell, we have a 12 year old Jonsered CS2150 (equivalent of HVA 350) plastic or farmers/homeowners saw, which is still running like a perfectly new saw. It's light and powerful, only the plastic chaincatcher of the early version has been knocked off, all else is fine with it. Two nuts holding the chain cover and tightening the bar on the CS2150, but I am not overtightening the nuts and keeping the bar at the standard length of 13" it came with.