British Postmark: What Type Is This?

I don't find the book 'Collect British Postmarks' a very satisfactory guide to British postmarks. This one, for example, is obviously from Bethnal Green, but there are a number of suffixes that are not listed in the book: "IS.0I.E". This has me baffled. Does anyone know what kind of post office was signified by these suffixes?

The postmark may be related to the railway as there was more than one post office in Bethnal Green and one could have been located at the railway station and would have been a Railway Sub Office.
The name Railway Sub Office (RSO) was abolished in 1905 and they then became Independent Sub-offices until 1909 when they became Post Towns. However the CDS postmark remained in use for many years afterwards.

BETHNAL GREEN SO. 1 E 4
SO = Sorting Office.
You may be reading the last leg of the "N" as a 1