It is the Montblanc 88, a gold-plated ballpoint that, according to Collectible Stars (page 59), was produced between 1961 and 1970. By contrast, the 98 consists of solid gold and was made in 14- and 18-carat variants. The standard work Collectible Stars is available via collectiblestars.de and provides further details. Original refills for the 88 are no longer delivered by Montblanc; alternative refill options have to be procured via collector sources.