See attachment of some photos of 15cm and 21cm Krupp Morsers from 1880s. The surviving 15cm Morser is from the History Museum of Pacific Fleet in Vladivostok, which I believe was used by Chinese army in Port Arthur before 1895 and later recycled by the Russians. For 12cm Morser I havent seen photos but as the barrel length is similar I doubt if they could be much different.
I would agree this is a Krupp piece. The ring counter-weight forward of the trunnions was likely installed for it's new roll a a mobile piece. ( perhaps 12cm ?) You will note the carriages in the first two pics are similar in design yet different in detail. The small diameter 8 spoke wheels don't lend themselves to serious road travel either. An expedient example for sure, and most interesting. Thanks for posting.
This was displayed at the Chicago Exhibition of 1892. The text says it was a replacement for the 21cm Morser.
The basic carriage and controls look like developments of the earlier fortress Morsers.
The muzzle velocity was quoted as 200 m/sec and max. range of 4000m.
The BOCN people (http://www.bocn.co.uk/vbforum/downloads.php) have a Krupp catalogue from the 1880s in their downloads but I don't think I've posted enough on their forum to download it. Anyone help?
Try to search "Krupp" on http://bibliotecavirtualdefensa.es/ , there are two sets of Krupp albums from 1892 and 1896 respectively. In the 1892 albums there are a number of 1880s Krupp Morsers, among which I have just found the 12cm Morser and the 24cm Morser that you've mentioned.
There are certainly more Krupp albums out there on the Internet, but this one seem to cover the most Morsers.