'screened' Hand Stamp On Airmail Cover From South Africa To UK

I recently came into possesion of this cover mailed from Phalaborwa, South Africa to the UK, looks like 1979. I have not come across the 'Screened' handstamp before. Does anybody have any ideas? Was it applied in SA or the UK?
There is nothing on the back of the cover other than the return address.

While not a specialist in SA or UK marking I will remind you that this marking could have been applied at the recipient's offices since it was going to HR.

Edit: HR = Human Resources = Personnel

I agree with the direction parcelpostguy is taking, that this mark is privately applied. Many companies discourage employees getting personal mail at work and will screen their incoming mail. This one apparently passed through as appropriate, but the "screened" would explain why it was (likely) opened before the recipient got it. And in the case of the personnel office, to eliminate any improper hiring influence. The other typical path from the mail room screening process would be a privately-applied "return to sender" mark with additional text along the lines of "this address not for personal mail".

The suggestion would be that any employee sending a letter to this company's HR department or any job applicant may have its letter screened before the HR department reads it. I would expect the HR department to execute the screening, which makes little sense for a letter directed to that department.

It is more likely the mail item contained some official document sent by a newly hired employee needed for a personnel screening. The 'screened' handstamp may have been applied to indicate the employee of this financial company passed the screening.

The other - probably the main - purpose of "screening" is to identify letter bombs etc, so in this case you could have a routine stamp to show that this had been through the process. I recall an instance at work when metal set off the screening alarm. This proved to be the staples on a package addressed to the Secretary of State that contained a dead fish.

Hmm, very interesting. It might be any of these or none of these!

Observations:

If my task were examining ("screening") the contents of the envelope for a new hire, I would mark the document inside rather than on the cover which could easily be separated and the rubber stamp would likely contain a line for initials/date to ID the screener.

Bombs: What is the historical context of letter bombs in the pre-1980s in the UK? Would this level of monitoring been done then?

At the time, there was the thread of IRA bombings. However, bomb letters, I think, are of a later period.
Still, this financial institution may have had a good security protocol at the time.

The Irish Republican Army was using letter-bombs in the 1970s. Hence, the "I won't open letter bombs for you" reference in the Clash's Career Opportunities (1977 for the non-musical).

Thanks for the feedback guys. After some thought, I too ruled SA as to where the hand stamp was applied. from what I remember all auxilliary markings were in both English and Afrikaans. I had also dabbled with the idea of a military screen as Phalaborwa was home to the 7th South African Infantry Division but then realized that the postmark was a year too early, and again, does not follow any known SA military markings that I have come across. The only other contenders would be the NIA or NIS but then, hardly anybody knew about them until they broke down your front door, so it kinda defeats the purose of them advertising the fact that they went through your correspondence.