Thanks for the expanded info
ExXB, I have to warn you that your anorak can be seen leaping out of the cupboard.
The other early user of the 74SP was South African Airways. Because they were not allowed to overfly Africa, due to apartheid, they had to fly West around the 'bulge' and not having to stop at Ilha doSol (Canary Islands) SID [not to be confused with SYD!] for a tech stop was very important to them. I recall stopping at SID several times with SAA 747-100 in the middle of the night and we were not allowed off it was just fuel. What flight crew call, "A splash and dash". Every business has it's own jargon!
As to codes, these pages may be useful:
International Civil Aviation Organization airport code - Wikipedia, the free encyclopedia
International Air Transport Association airport code - Wikipedia, the free encyclopedia
Airline codes - Wikipedia, the free encyclopedia
Mostly, it is flight crew (FC) that use the the four letter codes (LHR = EGLL, MAN = EGCC) and cabin crew (CC) that use the three letters, because that's what Pax are used to and Pax do not know about the other set. As
ExXB said, it's all about the long history of the business and everyone is now used to their particular set of codes.