If it turns out to be an old frame gun, the market value would be higher than what I mentioned above, even though a 150 prefix new frame Speed Six is probably significantly rarer.
The rarity of the old frame Speed Six is that the Speed Six did not debut until about mid-150-range and was done before 151 and most old frame guns were Security Sixes (some with fixed sights, later renamed Police Service Six in the new frame). So there were likely only been a few thousand old frame Speed Sixes, period.
The rarity of the new frame w/150 prefix is simply due to the prefix which has a thin appeal to collectors like me but it is the same basic variant as a 151 prefix and higher (up until the Warning appeared, excluding the Liberty models) Speed Six. So there were probably several tens of thousands of that variation made - but likely only a few hundred in the 150 prefix.
In the early days of the -Six, the fixed sight guns were restricted to LE sales, so were hard to come by new to a private buyer. Then, because many LE guns never see the free market when disposed of, they continue to be rather scarce as used guns. But Ruger dropped the artificial LE-only restriction in the mid-late 70's after their production had caught up to demand.
