I'm no expert which is why I hesitated earlier but looking at the green RCTS book 8A, I have this info. I don't know if it helps.
68891 was built in February 1914. It became an LNER J51/1 with 3 coal rails made from flat strips. It received its LNER number 3158 in October 1925. It was rebuilt as a J50/1 in March 1935 with a short bunker with cage-type coal rails. Its number was changed to 3181 in June 1945, 8891 in November 1946 and its BR number 68891 in October 1949. It was withdrawn in July 1961.
68894 was built in February 1914 It became an LNER J51/1. It received its LNER number 3161 in January 1925. It was rebuilt as a J50/1 in September 1932 with a short bunker with cage-type coal rails. Its number was changed to 3184 in June 1945, 8894 in June 1946 and its BR number 68894 in April 1949. It was withdrawn in September 1961.
68920 was built in October 1922. It became an LNER J50/2. It received its LNER number 3221 in January 1926, changing to 8920 in April 1946 and its BR number 68920 in February 1950. J50/2s were fitted with backing plates to their coal rails from 1931 onwards. 689209 was withdrawn in July 1961.
I think Hornby will be producing all types. See here and you can see rear views. http://www.ehattons.com/stocklist/SiteResults.aspx?searchfield=Hornby%20J50
If you're talking about 1948, all three would be in LNER unlined black with their LNER numbers because as you can see above, they didn't get BR numbers until 1949/1950.
It depends how accurate you want to be as the models you chose might have differences, eg the number of lamp brackets. There's been a debate on http://www.lner.info/forums/viewtopic.php?f=3&t=10879&hilit=J50s about this as some have three and others, five.