This Space Marine Terminator was the first metal Terminator Librarian, released in the RTB09 box set! He’s part number RTB/6 according to the 1991 Blue catalogue. He’s a 1989 Jes Goodwin sculpt.
He can be seen in White Dwarf #117 painted as a Space Wolf:
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Does the model have a special model bolter arm compared to other terminators ?
Not as far as I am aware – I found a copy of the RTB9 box set in like-new condition and it had just a regular bolter arm in there for him.