In someone's mind they may be completely logical.
They are simply catalog designations. Since the 60-70's they have used a five digit number. They may seem random to a customer but have some meaning to Leica.
Ikea uses similar alpha catalog numbers because the founder was dyslexic and prefers(d) catalog letters/words. That tiny bit of useless information came from their company history.
Well, ELMAR for for the 5cm ELMAR is about as simple as it gets, and EKURZ (kurz = short or 3,5cm Elmar and EKURZKUP or coupled version), ELANG (lang = long or 9cm Elmar) and EFERN (fern = far or 13,5cm Elmar) are not hard to understand either.
Likewise HEKTOR (5cm Hektor) or SUMAR (5cm Summar). Then you have the 73 Hektor and 135 Hektor (HEKON and HEFAR, clearly derivatives of HEKTOR), though HOOPY (28 Hektor) is harder to follow.
In finders, VIDEO (Latin 'I see') for the 35-50-90-135 'torpedo' in 1932 makes sense, after which VISOR, VISAX, VISET, VISIL are VI-derivatives, as are VIDOM and VIOOH.
My German is not up to making sense of many of these, buit it's not hard to imagine it as a pub game at lunchtime. There are probably puns, in-jokes, rude jokes, nicknames and contemporary references in many of them. And some, no doubt. simply
are meaningless.
Cheers,
R.