Peter D Gregory has commented:
I think the number is misquoted and in fact it should be SC7879E.
This would then make it a transfer from NBR area to GNSR area and allocated GNSR diagram 104S and NBR 100B AND LNER 3046.
There is a record of SC7879E fitting the above info which was originally NBR/LNER 32321
Comment from Paul B - this certainly appears to be similar to NB100B in Longworth, a 1914 built semi-corridor lavatory composite, but non of the numbers are similar.