This very small shunting diesel is another one of the mystery locomotives which may or may not have been actually used in Vietnam.
The only photos I have been able to find are those taken by a Vietnam Railways Forum member of a locomotive in a very poor state hidden at the back of a workshop (I believe at the Thai Nguyen Steel Works).

Deutches Reichban (DR-East German Railways) built 249 of these tiny shunting locomotives in the 60's and 70's. They were based on an original machine built in 1958 by LKM, Babelsberg, German Democratic Republic. Apparently some 20 'prototypes' were built before the full production run. The locomotives were not permitted on the main line as they were too slow, so were used as station shunters and within the grounds of manufacturing works where the did excellent work despite their small size. When no longer required by the re-united Germany they found homes in many industrial sites.

This locomotive was probably supplied by East Germany or another Eastern European country, possibly when the BR52s were supplied, and appears to be standard gauge. There only appears to have been one supplied. Whether it was supplied directly to Thai Nguyen or passed on by the Vietnamese Government is not known.

It appears to have been originally painted a light blue colour which I believe was a fairly standard colour for these locomotives.

This locomotive would make another interesting exhibit at a Vietnamese Railway Museum if one is ever established and if this machine still exists.

Steam Locomotives generally used the traditional French classification system. A steam locomotive with a 2-8-2 wheel arrangement is classified as 141 class, a locomotive with  4-6-2 wheel arrangement is classified as 231 class etc. Pretty straight forward, except what happened  if two classes had the same wheel arrangement? It would appear that there would be several different say 231 classes eg. 231-301 and 231-501.

Diesel locomotives have a completely dfferent clasification system.

In the south diesels were classified again by wheels, so BB for bo-bo types. The same system as used in France and still used in Cambodia. In the north I'm not sure.

Now all diesel classes start with the letter 'D'  (for diesel) then a number which I believe is related to the locomotives  power output. The third character defines  Electric' (E) or hydraulic  (H).

e.g. D5H class = Diesel + 500 hp + hydraulic.

Again, what happens if two classes are the same I don't know.

Therefore it would appear that metre guage locomotives can have the same class but not the same road number.

Standard guage locomotives have a slightly different system. Diesel sometimes have 'er' added to the class e.g. D19er, but sometimes don't, e.g. D14E. Steam?