I think the South African guns had WB before the number. It will be a commonwealth country perhaps Australia 🤓. That 5 digit number (21431)on my 90 is a NSW rego number. I’m not sure if New Zealand require Airguns to be registered. 
I am eagerly awaiting the arrival of a 70 I purchased a few days ago. It’s .177 and I think beech stock but I’ve been assured it’s in good condition. I’ll pop up a picture and chrono results to compare when I have it. 

Did you change to a synthetic piston seal on your rebuild?
 
Have you noticed that the numbers on Falke 70 #14302 and Falke 90 #21413 are not very well aligned, as if stamped by hand, and also that the same die sets are used to stamp the numbers on each gun? Perhaps both these guns were made for export, to markets which required a serial number, and hand stamped at the factory before shipment? 

At risk of seeming to fixate on "fit or assembly" numbers on the Falke 80/90, I happen to be restoring a Weihrauch HW55MM from 1961; almost every small part has a number that relates to the formal serial number of the gun stamped on the LHS of the tube. I'm not sure if this is true of the later HW55, but the HW55 was competition with the Falke in that era, presumably hand fit and assembled as I surmise is also true of the Falke? In both situations, fit numbers were required for reassembly after blue?

Finally, a machinist friend tells me that it's very difficult to accurately hand stamp a number on a curved surface. Maybe 80/90 don't have typical serial numbers because there are hardly any flat surfaces, it's a small company, and they don't have the tooling available to Weihrauch/FWB/Diana?
 
Only the “4” on my 90 is a factory stamp. It is number 4 and the other numbers were put on by the New South Wales police department for registration. 
On the 80 they registered the rifle by its factory number “64”

The 70’s and all the Falke break barrels were not stamped at the factory. So those numbers on the 70 are all hand stamped in a country that required the number for registration before it made its way to the US. 
I would love to see some pictures of your HW 55

I have some ideas on the Falke numbers and I think it’s all making more sense. I post more of an explanation soon. 

 
Here are fit/assembly numbers from a HW55 "Meister und Match" from about 1960-61. As with the Falke, only the most critical parts are stamped, after fitment, with the gun in the white, before blue and final assembly? 
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