The last of the outside framed classes had been delivered to the Rhymney in 1900, and from then on the locomotives took on a much more modern appearance. The first were what was later to be called the M class, and the detailed design is usually credited to Robert Stephenson & Co. All subsequent locomotives for the Rhymney, other than a pair of locomotives which started life as railmotor units, bore a distinct family resemblance. These locomotives form a complicated and rather incestuous group, since around 17 variations on the 0-6-2 theme can be identified, all sharing a common 7ft 3in + 8ft 0in + 6ft 0in wheelbase which was also seen on similar locomotives for other lines and even the GWR's 56xx class. Within that there are three basic themes: 4'6 wheels with larger boilers (M, R), 5ft wheel with a smaller firebox (P), and 4'4.5 in wheel also with the smaller boiler (A). There were subclasses with round top and belpaire firebox boilers, and some experiments were made with boilers swapped between classes. When the GWR introduced their own boilers the Ms and Rs received Std 2s, and the Ps and As Std 10. It's also helpful to consider the S and S1 class 0-6-0T when looking at the RR locomotive development. The actual construction order was M, R, S, P, A, A1, P1, S1, AR/AP, and its helpful to keep this in mind tracing the development between classes.
The original boilers on the Ms, which had Belpaire fireboxes, were considered unsatisfactory, and by the grouping all were running boilers of the design first used by the R class. The GWR tended to treat the Ms as one class with the later Rs, albeit with subclasses represented with different diagrams. They were numbered 33 and 47-51.
This first sketch represents what was temporarily known as the Mr Class, being the M class fitted with the R class boiler. Once all had been converted the r disappeared again.
The Ms received R class cylinders in the 1930s. One was rebuilt with a Standard 2 boiler and GWR style cab, but otherwise they had few changes beyond safety valves and larger bunkers. The second sketch is intended to show the one M, no 47, that received a GWR boiler - a standard 2. The resemblance to the GWR 56s is surely no coincidence, but in point of fact the conversion wasn't carried out until after the 56s had been in service a few years.
Two Ms were withdrawn in the 1930s, but the rest survived the war, three getting to British Railways and the last going in 1951.
The dark Brunswick green I've chosen to approximate (with lining missing) RR paint tends to hide detail doesn't it. I shall have to think about that.