Possible location, Via Ugo Bassi, 32

Sam and companions travel through Bologna, by train, enroute to Florence from Venice. Mark Twain Project: Quaker City Itinerary

Bædeker Northern Italy (1877) Route 43 page 270

Settignano is a frazione on a hillside northeast of Florence, Italy, with views that have attracted American expatriates for generations. The little borgo of Settignano carries a familiar name for having produced three sculptors of the Florentine Renaissance, Desiderio da Settignano and the Gamberini brothers, better known as Bernardo Rossellino and Antonio Rossellino.

The Balearic Islands  are an archipelago of islands in Spain in the western Mediterranean Sea, near the eastern coast of the Iberian Peninsula. The four largest islands are Mallorca, Menorca, Ibiza, and Formentera. Many minor islands and islets are close to the larger islands, including Cabrera, Dragonera, and S'Espalmador. 

The Quaker City made port there July 14, 1867 and Mark Twain spent two days sight seeing before taking the train to Alessandria and Milan. The ship stayed in port nine days before sailing to Leghorn. Twain also passed through this city en route to Florence, with his family in November of 1903 and sailed from there with his daughters after Livy died the following June.

From Baedeker:

Bædeker Northern Italy (1877) Route 20 page 118

May 3 & 4, 1878

 We made a short halt at Frankfort-on-the-Main, and found it an interesting city. I would have liked to visit the birthplace of Gutenburg, but it could not be done, as no memorandum of the site of the house has been kept. So we spent an hour in the Goethe mansion instead. The city permits this house to belong to private parties, instead of gracing and dignifying herself with the honor of possessing and protecting it.

Homburg is located in the northern part of the Saarpfalz district, bordering Rhineland-Palatinate. It is 16 km from Neunkirchen and 36 km from Saarbrücken.

The city districts are situated in the Blies valley or on its tributaries Erbach, Lambsbach and Schwarzbach.

Homburg is composed of Homburg center and nine city districts: Beeden, Bruchhof-Sanddorf, Einöd, Erbach, Jägersburg, Kirrberg, Reiskirchen, Schwarzenbach and Wörschweiler.

Cadenabbia (Cadenabbia di Griante) is a small community in Lombardy, Italy, in the province of Como, on the west shore of Lake Como.

Sam and companions arrive in Rome, by train from Civitavecchia, July 27, 1867. The date is uncertain but Sam probably left Rome, by train for Naples, on August 1, 1867. Mark Twain Project: Quaker City Itinerary

Bædeker Italy, Central Italy and Rome (1867) Route 12 page 86