I have done a little research on Google, and the locomotive in the sulphide image is very similar to some French locomotives of the 1850-1860 period. Below is an 'improved' image of the sulphide, and an image of a Paris-Orleans railway 0-6-0 well tank from the 1850s. Similar in many respects, but not identical. The cab is noticeably different, but the commonest design of cab at that time in France looks like the one in the sulphide.
So I think this is a French (?maybe Belgian) locomotive. Why was the sulphide made? It is nothing special as a locomotive, so maybe to commemorate a particular event, or anniversary? Perhaps a 50th anniversary? It could be that the Ysarts were responsible for the weight, having been in France and got hold of the sulphide: but I would think it more likely it was made in France or Belgium.
And maybe that is where the Ysarts got the idea for this style of weight......as well as the harlequin designs.....