"Wills Inventories, and Death Duties" by Jane Cox "
for a nineteen page list of terms and abbreviations used in all types of probate documents." (not seen and not my description)
Jane Cox also wrote
"The Records of the Prerogative Court of Canterbury and the Death Duty Registers" Published by the Public Record Office 1980, 40 pages
Whether they explain anymore & NE I don't know.
The only 3 books I have mentioning Death Duty are:-
1. Guide to the Contents of the Public Record Office. Volume III. Documents Transferred 1960-66, Crown Copyright HMSO 1968
Which says IR 27 lists Wills Proved and Administrations Granted
2. Tracing your Ancestors in the Public Record Office by Jane Cox and Timothy Padfield Crown Copyright 1983, 2nd Imp 1985
The Death Duty Registers (IR 26, IR 27) are very useful as a means of discovering where a will was proved or administration granted, and thus where the records now are. ... Not all Wills, however, are included as some estates were of insufficient value to attract tax.The document you are after is listed in IR 27.
3. Tracing Your Family History
By Anthony Adolph
He seems to suggest also trying the most Local Court and suggests
Probate Jurisdictions — Where to Look for Wills - Jeremy Gibson & Else Churchill
I have read tonight London had about 6 Courts dealing with Wills
York had more than 2 Courts for Wills, because I had this same difficulty with the Borthwick who only seem to have 2 searchable database Calendars and they had to do a manual search and found the Registered Will I wanted.
I feel you ought to email Lambeth Palace Library, explain and ask them? There is a drop down and highlight Wills
https://www.lambethpalacelibrary.info/using-the-library/enquiry-form/ ----------
The National Archives 2004 Information Sheets & Links 2004.
Probate Records Legal Records Information 23
https://web.archive.org/web/20090216141958/http://www.nationalarchives.gov.uk/catalogue/rdleaflet.asp?sLeafletID=168&j=1Some Abbreviations here, but not NE!
8. Abbreviations Found in Various Columns of the Death Duty Registers
https://web.archive.org/web/20090322235944/https://www.nationalarchives.gov.uk/catalogue/rdleaflet.asp?sLeafletURL=http%3A%2F%2Fwww%2Enationalarchives%2Egov%2Euk%2Fcatalogue%2Fleaflets%2Fri2165%2Ehtm&lBack=-1&j=1Mark