For the whole building, see
Image and
Image The stone is set above a fire exit on the eastern side of the building.
[UPDATE, AUGUST 2016: I found that Dumbarton Library had a pile of printouts, featuring only my picture from this page, and a very much cut-down version of my accompanying text from this page, with no credit to me for either. I choose to mention this here because library staff, if anyone, ought to be thoroughly acquainted with the legalities of copyright and licensing, and should be setting a better example.]
The stone bears the text "Tu Des Corona Decus", and the dates 1732 and 1790. The stone came from the Mackenzie House (the townhouse of the Mackenzies of Caldarvan, on whom see
Image). The Mackenzie House stood in the High Street, and 1732 and 1790 are the dates when it was built and enlarged, respectively. It was demolished in 1907; see
Image for more information.
The stone is said to have originally come from St Mary's Collegiate Church, of which the most prominent remnant is
Image 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; see
Image for other examples (that item contains a link to a page with a picture of the Mackenzie House; in that picture, the stone shown in the present photograph can be seen above the central dormer window).
A good description of this stone can be found in Donald MacLeod's "Dumbarton Ancient and Modern" (1881), a large-format book, of which only 200 copies were printed.
In the section entitled "Old Tolbuith and Mackenzie's House", the book discusses the three dormer windows of the Mackenzie House. Of these, it says that "over the one to the left there is as finial a somewhat grotesque figure of a diminutive man or boy in sitting posture, having a round flat bonnet on his head. Over the imposing central window there is a Latin cross, crowned by a mitre, on one side of which there is a Scotch thistle, and on the other side a fleur-de-lys, all admirably sculptured, beneath which there is this inscription:– "Tu des, corona decus"(*), "Do thou give me glory for a crown". The eastmost of the dormer windows has for finial a small globular shaped stone, which looks as if it had formed a portion of a pinnacle of the 'Auld Colledge'."
[(*) A personal observation: that Latin phrase seem ungrammatical.]
By "Auld Colledge", the author means the Collegiate Church. The facing page of the book features an illustration of the Mackenzie House, with this stone visible above the central dormer window. The old tolbooth appears in the same illustration; the Mackenzie House was on the north side of the High Street, and the old tolbooth was immediately next to it on the left (west). Readers can examine the same picture online: see the "Ancient stones, Glencairn's Greit House" link given above; that item has a link to the picture.