Unfortunately Old Maps became a subscription site due to mis-use by commercial interests.
Amongst the NLS town plans series there is no early plan for Merthyr Tydfil at 1:1056, c1850. Those were only produced for larger places. The only plan is 1:500, 1875, the same as the National Library of Wales. It is a good plan but involves a lot of sheets, so tedious if you do not know approximately where to look.
Correction. This is the georeferenced layer for that.
https://maps.nls.uk/geo/explore/#zoom=14.0&lat=51.74802&lon=-3.37527&layers=117746211&b=ESRIWorld&o=100The NLS older street index is derived from the much smaller 6-inch scale, c1900, and I have found some streets are not named in that, in busy areas.
The Old Maps scans are those done by Landmark from the OS's own files, during the 1990s I think, and are not such good resolution as the NLS scans.
So it is swings and roundabouts!