Early references are have six digits. Most late 50s and early 60s references have 5 and a further digit that differentiates executions or variations within the same reference. Later 60s and 70s references have six digits and a further digit as explained above.
Reference 224106 (Which I would interpret as a 40s case reference) would be completely different to reference 22419-6 (a 50s reference which I would expect to have six different executions)
There are exceptions to the rule ....
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