What a mark on a document actually says
Strip away the wax and the wording and each of these marks makes a single, small claim: this signature is real, and it belongs to that named office. Nothing more. The mark does not read the sentence above it, does not know why you need the paper, and takes no position on whether the record is current. Grasping that narrowness early saves a great deal of misplaced hope.
Where Mauritania stands
Mauritania has not joined the Hague Apostille Convention, so papers intended for Mauritania need embassy or consular legalisation instead. There is no apostille issued in Belfast for use in Mauritania that a desk there will act on. What changes on the longer road is the number of marks collected, not what any single one of them promises about your document.
The question nobody in the chain asks
At no point does anyone ask whether the paper is right. The register office recorded what it was told at the time. Each certifying stage looks at a signature. The office at the far end reads the finished bundle and acts on it. An apostille from United Kingdom to Mauritania is not on offer, but even where that short road exists it changes none of this — the number of desks alters nothing about what gets examined.
Which makes the reading your job
That leaves one real piece of quality control, and it is yours. Read every line before the sheet leaves you: names in full, dates in the order you expect, the issuing office named clearly at the foot. Compare the spelling against the passport or the file the paper will sit beside. Ten minutes here is worth more than every stamp that follows, because the stamps cannot see what you are looking at.
Where a clean Belfast record comes from
Northern Ireland runs its own civil register, so a birth, marriage or death recorded here is issued by the General Register Office for Northern Ireland at Colby House, Stranmillis Court, Belfast BT9 5RR rather than by an office in Great Britain. Ordering a certified copy from there gives you a current, legible sheet to read carefully and to send onward, while the one you have kept for years stays out of the post entirely.
The office people mean by the short road
The FCDO Legalisation Office at PO Box 6255, Milton Keynes MK10 1XX issues apostilles for the whole United Kingdom, by post, with no counter anywhere. Searches for what you need for an apostille for Mauritania point at that office, which is the right one for many destinations and the wrong turning for this one. Its charge is still a useful floor when you are working out what the correct road will run to.