This appears in an early picture of the Tolbooth so is included here.
Lairich Rig has recognised the gable as being the feature that has been built in above the side entrance of Dumbarton Library and gives us some more information. The Mackenzie House, built in 1732, enlarged in 1790, and demolished in 1907, was the townhouse of the Mackenzies of Caldarvan. In the engraving mentioned above, a distinctive stone can be seen above the central dormer window of the house. [Lairich Rig].
Lairich, who uses a number of written sources goes on to say : The stone is said to have originally come from St Mary's Collegiate Church, of which the most prominent remnant is NS3975 : The College Bow. Smaller fragments of the Collegiate Church survive; some, like the one shown in the present photograph, have been incorporated into the fabric of later buildings.
A close-up of the gable in its original setting.
The gable as set into the extended section of Dumbarton Library.
GEOGRAPH - Lairich Rig. This gives some very interesing information on the town at this point. https://www.geograph.org.uk/article/Dumbarton-Prison