I have never seen tetraethyl lead/lead bromide deposit rust oxide on plugs in my life at any AFR ratio. Have dozens of cars running it and plugs just don't come out looking like that (but could always be a first for it I guess).
http://www.dansmc.com/Spark_Plugs/Sp...s_catalog.html
I say you had water in that fuel or somewhere in your system. Or extreme overheating from lean running? though again I have seen engine destroy themselves and spark plugs shatter in rotaries (or in NA misfiring due to being to lean) before generating such heavy deposits like that pictured in such a short space of time.