A bit of info, taken from Yeadon's Register:
As far as I can tell, only three of the original N7/GEs (9602, 9609 and 9618) still had Belpaire fireboxes at Nationalisation, and all three were exchanged for roundtops before 1949 was out. I don't know if any received BRITISH RAILWAYS or early crest branding.
Of the 46 N7/1s and N7/2s still with Belpaire fireboxes at Nationalisation, 17 had lost them by the end of 1950 and a further 15 by the end of 1953, leaving 14 still with them.
These, with the dates they lost them, and their allocation when they did so, were:
69624 12/55 30A
69626 4/54 30A
69627 9/56 30A
69629 4/56 34B
69631 3/55 34C
69637 10/55 34C
69645 5/55 30A
69646 5/54 30A
69662 2/55 30A
69683 12/54 30A
69689 3/57 32D
69690 1/55 32A
69694 11/54 37B
69695 12/58 30A
Hope that's useful. Apologies for any mistakes.