I found this on a vintage pen site:
136 War Time Meisterstuck
With the vintage 136 War Time fountain pen, collectors will have a writing unit that measures 5 1/8 inches. In addition, it features a rare palladium nib and is recognizable by its black body and gold clip.
Many thanks for all of your comments! It is indeed a lovely pen but will need some restoration. Thanks for the images Stevesurf. There are certainly lots of similarities between the pen I have and those in the pictures. A couple of minor differences are that my pen has a slightly different shaped clip to the one in the pictures - as mentioned above, the pen I have has a clip the same as the 135 in the Pinterest link in my first post here. Also, my pen has a gold band at the bottom of the cap too. One question (I'm a MB novice!) - is the longer number on the barrel a serial number? Is this a different number on every individual pen? The number on my pen is different to the one in this picture.
I'd love to have the pen restored and have emailed Montblanc for advice about this. Whether this is a 135 or 136, does anybody have any idea regarding value, please? As it was with my great grandfathers papers, my father will no doubt restore the pen to keep it but we want to know if it's something we need to look at insuring etc. All the sites I've looked at vary in terms of valuation for these types of pen (135/6). Many thanks - everyone here has been so helpful and encouraging :-)