A few random thoughts:

Keelung is a romanized version of that city’s name as pronounced in Taiwanese (the closest approximation in pinyin would be “Gailang”. I don’t know whether that name is itself a “Minnanized” version of a previous aboriginal name.

Peking is what the characters 北京 sounded like to a British postal worker when pronounced by a Cantonese speaker in Hong Kong or Guangdong sometime in the 18th century. If you get someone to say those characters in Taiwanese it also sounds a lot like “Peking”.

The Postal System isn’t really a system. It’s probably better thought of as the practice of using romanized forms that had become traditional by the late 19th century. Often, these had nothing to do with Mandarin (e.g., Canton for what is now labeled Guangzhou); in this one particular aspect the Postal System has a distinct advantage: the spellings of place names were generally assigned on the basis of the language of the people living in that place instead of the present Mandarin uber alles approach.

I don’t know anything about the etymology of Jilong/Keelung. But I suspect it is like so many other place names, where the name comes from the name given to the place by the original inhabitants (a certain tribe aborigines, in this case). The proper pronunciation probably got mangled some whenever whoever it was from outside (certainly not a trained linguist!) wrote down the name. Then Chinese characters — often with a dash or two of auspiciousness added — could be assigned based on the pronunciation of the person who gets the job of officially recording the name. Then, finally, people start believing that the Chinese characters represent the real etymology, and so we end up with nonsense like “terraced bay” being the “real” meaning of Taiwan. And so we end up with “the common fallacy of wàngwénshēngyì 望文生義, whereby the semantic qualities of Chinese characters interfere with the real meanings of the terms that they are being used to transcribe phonetically.” (See Victor Mair’s How to Forget Your Mother Tongue and Remember Your National Language.)

BTW, do you know how the postal system’s crazier romanizations, i.e. Jilong->Keelung, came about?

Note, though, that “Peking” is Postal System, not Wade-Giles, in which it would be Pei-ching.

I have gotten some information on the romanizations in Taiwan. It might be confusing even for the Taiwanese to use several systems, and all the more so for foreigners. In this sense, I feel very comfortable living in China.

1. The “国語羅馬字”was published in 1923 in China. It was kind of an improved Wade system and did not become popular due to the introduction of the “中国拉丁化新文字”in 1929. In China, the Hanyu Pinyin came into wider use later.

2. But the “国語羅馬字” is still being used by some linguists in Taiwan. In this system, for example, “朱” is romanized as “chu” and “謝” as “Shieh.” And tones are represented by using alphabets, not by tone marks.

3. In 1986, the Taiwan government published the “simplified 国語羅馬字,” and it has been used for some street signs in the areas except in Taibei since 1998. This system uses the same tone marks as the Hanyu Pinyin in China.