This an early Montblanc Button Filler made in Germany in the late 1930's. I am not sure of the exact model but I am 100% certain it is genuine and original. The cap has the Montblanc logo and the steel nib has the Montblanc star with the number 4 1/2 inside.
Although Montblanc are not renowned for their button filling pens I have it on good authority that this type of filler was used in pre-war models, maybe to simplify production or maybe because of a shortage of materials for the piston fillers (brass was a major component in the piston pumps at the time and it may have been in short supply as materials were diverted to the military) who knows?
The pen itself is in good condition with minor marks and wear. The cap ring is chrome plated whilst the clip is gold plated so possibly the clip is a later addition. The blind cap is stamped B which is correct as all Montblancs of this era had the nib width stated in that place.
What is not in doubt is the quality of the nib, the steel Broad Italic writes as well as the best gold nibs and is an absolute pleasure to write with.