Have you actually tested them? They may be OK, just dirty.
I'm familiar with Rayburn (er) systems, and they are usually oil or gas industrial heat exchanger burners. (I spent a few years in a potato chip plant as plant engineer, and subsequently have run an industrial controls business)
The oil burners especially produce so much "gunk" it's often attracted by electrostatic charge and will occasionally make components look burned when they are in fact, only dirty. The pattern around the diodes looks like this effect.
I've seen a pair of series diodes used as a flame out indicator before as well, and wonder if that may be the case here. (they react rapidly to temperature changes). Does that board tab face the burner thru an opening in the burner box? If so that may be the function of the diodes. Rayburner always held onto old tech way past it's prime, as the diode flame detector was a very 1940-50s kind of technology. Then again I could be totally wrong with that assumption...
Hope I helped a little...
Scott