I am not sure. Devoto spears are heat treated and have the typical brown dull color like the Seatec. They don't reflect light and so they not as conspicuos as polished stainless steel. They are as hard as it gets I would assume. The hardness of the base steel I believe is limited by the machinability before heat treatment. To drill the line and pin holes can be very difficult.
The only thing the spears don't have that I think is desiderable is the single barb flush in the shaft (like Seatec's). I ask Mr Devoto why it does not make them and he said that they used to but stopped because of complains of breaking tips in way of the barb. I hope he will make them again.
Another thing I like are the shark fin notches. They are way smaller then most on the market and offer very little resistance. I modified mine with a a very thin grinding wheel to create a bigger seat for the 2mm Seatec Smoby wishbone or denema. I cut a bit into the shaft so I have the best of both worlds (sharkfin and cut shafts).
Without the modification holding a thick wishbone may prove difficult.
The slip tip I got is the one one page 3 of the catalogue
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I like the design with the cable passing through the treaded bushing.
Cheers
Giovanni