Spring length, Monster 696

[Mozella]

I was fooling around with the rear shock pre-load on my new (to me) 2009 Monster 696.  The manual says very little about it.  It gives a length of 150mm as the stock setting with the rear wheel off the ground.  It also says  "When adjusting the spring preload, do not exceed a spring length of 181 mm to avoid damaging the swing-arm."

That's the part I don't get.  It looks to me that if you exceed a length of about 167 mm the adjuster lock nut will hit the swing arm casting.  My spring is unmarked, but as far as I know the shock assembly is stock.   I haven't removed the spring yet, but I did slack the adjusters off as far as I could and it looks to me as though the free length is under 181 mm.  I was unable to fully unload the spring, but I would guess that the free length might be around 175mm or so.

Does anyone know the free length of the stock spring and/or can anyone shed light on that measurement of 181mm and why it's significant?