Sorry Jos. Just one answer and then more questions.
The AT-21 was part of a designation series (sort of). Dutch Wikipedia mentions other, unproduced Aviolanda target drone designs -- T-11, T-11A, and T-20.
Aviolanda also licence-built Snecma acoustic-type pulsejets as the 70 kg AS-11 (powering the AT-21 drone) and a [proposed?] more powerful AS-21.
SNECMA usually numbered or named its pulse-jet designs (eg: 3340, 5158, or Escopette, Ecrivisse). So, can we assume that the AS-xx series were also Aviolanda designations?
It's tempting to suggest that AS- was for Aviolanda-SNECMA. But I doubt that AT- is as simple as Aviolanda-Target. If it were, why first plain T- designations and then the AT-21?
Online mention of Aviolanda as designing the "Netherlands’s first UAV/ROA designed for tactical photography". Did the KLu actually field this UAV or did it remain a project> Was the 'air vehicle' a T- series or AT-21 spin-off? Or was the UAV yet another original Aviolanda design?
BTW, Hesham mentioned an original Aviolanda design for a pre-WWII fighter. The mock-up shows a gull-winged monoplane with a retractable gear. Perhaps this fighter was intended as a replacement for the Fokker D.XVII biplanes which Aviolanda had licenced?
Wikipedia describes this "Eigen jachtvlieguig" as follows:
"Aviolanda also tried to develop a fighter around 1938, but this is not further than a mock-up (1:1 presentation) without engine."
For those who want more detail on the Avioland AT-21 built between 1954 and 1958, Roskam's Airplane War Stories by Jan Roskam has lots of details about the AT-21 -- including a cutaway drawing (pg.73), photo of AT-21 on launcher (using the same RATO rockets as RNethN Sea Furies, pg.74), and 3-view sectional drawing (pg.76). Or see: