Please Help Me Identify This Stamp

Hello members please help me identify this stamp

This one with some discoloration ?

Mmm. Discolouration is an understatement! The stamp has been bleached or otherwise chemically treated to remove the green colour in the vignette. The phrase we use is "Does not represent the colour as originally issued". If the stamp has Multiple Crown CA watermark, it was printed on both ordinary and chalk-surfaced paper, and it would be interesting to know if the chalk surfacing survived the chemical treatment.

So, either SG 6 or 23, depending on watermark?

Or 23a, depending on whether the Multiple Script CA watermarked stamp has normal or chalk-surfaced paper.

What I find intriguing, is the Postmark.
Very unusual
There appears text under the Capital N or Z or whatever. (?ion)
Perhaps a Parcel strike?

Looks like 'MOMBASA'? The cancel is uncommon...

I say, marvelous work there, Perf 12.
You are a whizz.

Thank you everyone for your responses.
@perf12 Thanks for sharing the picture ....yes it's the same stamp of 4A.how much will be it's worth?

I'm afraid the stamp is not worth anything, because it has been so badly altered.

@Tim H thanks for your responce .The stamp that I have is same that posted by Perf12 .the alteration in color is due the picture is clicked by mobile and the color has altered a bit otherwise in real it is exactly the same as posted by Perf12.