"I think they mis-wrote the G - it should be a C:
A Catholic tradition for the Epiphany is the blessing of doors. With chalk the letters CMB are written, symbolic of the names of the 3 magi (Caspar, Melchior and Balthasar) and also an abbreviation for the Latin blessing Christus Mansionem Benedicat (Christ Bless this House). If your family is interested in this blessing, you can read more about it here. May each of your homes receive "health, purity, the strength of victory, humility, goodness and mercy" and many other blessings!"
With this new information in my arsenal, I returned to Google and found A BILLION (at least) listings confirming this to be the case. The 19 and the 89 surrounding the three letters represent the year of the blessing (1989). And I have also found an explanation for the G/C confusion - in Hungarian, "Caspar" is "Gaspar"... so the mystery is solved! Thanks, Ash!
Has me thinking about other ways to bless/honor/bring good luck upon a home...