There's this thing called "observability". On occasion IBM comes out with new hardware which requires a 'conversion' of the compiled code. Anything compiled prior to V5R1 and had the objects observability removed could not get converted and thus had to be recompiled. Removing observability was a thing people used to do. Back in the day it saved some disk space. It actually was a technique recommended by some back when disk drives were still described in MB (as opposed to GB or TB). Some believe that removing observability makes it tougher to reverse engineer compiled code.
CISC-to-RISC migrations (V3R2 and earlier to V3R6 and above) was the first time everyone went through this.
The next time was V5R4 and earlier to 6.1 and above.
Stuff compiled to V5R1 and above retains enough information to undergo conversion even if observability is removed.
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