Yes, it is a type aligning prism - used when the entire type-basket is being soldered in a jig during production.  Usually the whole basket has these - it is unusual to see it on only one typeface.  Perhaps this particular one is a later replacement ?

I believe it's an indicator to the slug's age. It seems to me that Triumph primarily used RaRo slugs, and earlier RaRo slugs, for example those from the '20s to the '50s, did not have the prism feature. However, RaRo slugs from the 60s onward did. I know of at least one foundry that offered both by the late '50s, so all of these years I'm throwing around are approximations. Of course it's possible that I'm wrong about all of this, but this is the premise I've used up to now.