Caroline Gurney <research@...>
There is a blanket 100 year closure rule on all the naturalisation application files in the series HO 405. Given that the files include police reports on individuals, that's understandable: https://discovery.nationalarchives.gov.uk/browse/r/h/C15109.
As Jill Whitehead has said, you can submit a Freedom of Information request to open this file - or at least obtain certain information from it, even if it cannot be fully opened. The link to do this is on the catalogue page you shared above.
These files were supposed to be closed when the person was either naturalised or died. So it's a mystery why a file on a person who died in 1978 remained open until 1996. You will not be able to find a naturalisation certificate if he was never naturalised. In submitting your FOI request, I suggest you specifically enquire whether the application resulted in his being naturalised.
The United Kingdom Visa and Immigration Service has records of those who were given indefinite leave to remain in the country, at any date. You can email: subjectaccessrequest@... to enquire about records they may have for E Guttmann.