Serial number puts that thing at ~1975. That would be right on the verge of whether target grips would have been standard or not. Beings as your gun does not have a target width trigger and is a 3.5 barrel gun, I think it is more likely that it shipped with magnas than it is that it shipped with target grips. The previous owner making the decision to swap the grips for larger target sized siles would also support the theory of it shipping with magnas. All that being said, it wouldn't surprise me one bit if it originally came with target grips as I believe late 1975 is when target sized grips became standard configuration on the model 27. If you intend to shoot this thing on a regular basis and prefer target sized grips, I wouldn't think twice about doing so and I wouldn't at all feel that it is an incorrect representation of the gun regardless of what it shipped with.
If you were to elect to put era correct target grips on it, goncalo alves silver washer football targets such as the ones on CH4's gun would be correct. I would also consider era correct smooth targets for this gun, due to a personal preference for smooth grips.