A 136 with a silver overlay calls for considerable caution. Collectors report that Montblanc never offered this model with a mixture of silver and gold parts; far more likely is that an ordinary 136 was fitted afterwards with an overlay from another pen or one made specially for it — in collectors' jargon, a cobbled-together piece.
The hallmark supplies a further clue: German pens from the period when the 136 was made normally used 900 silver, so a 925 hallmark points to an overlay produced later rather than to a factory version. Material combinations that appear in no catalogue version are to be judged in the same way, as are fits at joints and rings that are not quite true. For collector value this matters a great deal: even neatly executed overlays added after the fact reduce the value considerably, because the pen is no longer original. A reliable judgement requires the pen in hand; photographs from a sales listing are rarely enough.