Such incompletely filled-in warranty cards often come from internal employee sales in which MB employees acquire writing instruments at special conditions and partly resell them, which is observable above all in November and December on eBay. Since Montblanc by the serial number typically captures only the manufacturing date, not the purchase date, cards without entered model and clip-ring number theoretically suit several writing instruments. Precisely for this reason they can be misused for sale together with counterfeits, especially since blank warranty cards and packagings are at times auctioned on eBay. By now, warranty cards themselves are also being counterfeited, so even filled-in cards are no reliable authenticity proof. Certainty offers ultimately only purchase from specialist trade or from known sellers.