Sadly the S1MS was a manifold of poor materials, poor design and poor performance. Early well intentioned attempts at enhancing its performance were ineffectual and it was quickly superseded by a sequence of similar, but improved, dual plane manifolds by Shelby and the aftermarket. This appears to be detritus from that period.
The S1MS was an Offenhauser gravity sand casting of poor aluminum integrity, also the design was not robusted to compensate for this poor materials quality. It appears the posted intake has had its rear ears broken off and ‘repaired’. This is sadly common and that design is in no mans land at 10-15 ftlbs tq. Expert repair and reskinning is available ($$), but then what do you have?
The holes have been enlarged / hogged out so be careful on port alignment and barely snug those last two bolts!
On a 260 this intake may well diminish that engine’s efficiency.
The S1MS does live on in lore though, and it is of value to a restorer uninterested in performance.
(If on a budget there are a lots of F4Bs cheaply available)
Same intake broken and repaired/reskinned: