This first miniature was from the First Warhammer 40k releases March 87 flyer See flyer here he is listed as an Imperial Bodyguard. This miniature reminds me of one of the 2000AD comic character Torquemada Terminiators he wears a similar antifaz steeple hood travelling the galaxy in search of Xenos. (very Ku Klux Klan like)
In the first edition rule book you can see a picture of him guarding the Emperor. See below.
And Imperial Bodyguards with an Astropath here, I think the artist used the citadel miniature as reference for his drawing.
Imperial Bodyguard escorting what looks like a Tech Priest.
I hope you enjoyJ