Some shotguns were made in England with straight grooves as well. The idea was that with the correct sized shot, they would create a better pattern.
They did shoot well , but were heavier than a normally bored sporting gun, so were not in fashion for very long.
According to Geo. Shumway, they were quite popular (straight rifled barrels) for boar hunting in Germanic lands.
Whether this one is for shot, or for ball for boar I have no clue. Would have thought a rear sight for boar.