Going back to my first bullet.
It was quite common back in 709x machine days to alter TRA instructions (an unconditional branch) to a NOP instruction as a programmed switch. This practice, which you are sort of doing here, was avoided in System/360 by using TM followed by a BC instruction.
Early versions of HASP for OS/360 persisted in altering the second byte of instructions like MVC, CLC or TR to set their lengths. HASP Version 4 for OS/VS2 Release 1 replaced all of this usage with an EX r,... instructions.
I read through your post again. I conclude what you think you are doing is testing a field to see if it contains EBCDIC characters A through R (but not S through Z) or 0 through 9. Your definition is not exactly that, but I think I have the flavor. If that's the case, most Assembler programmers would write something like -
PTABLE DC 256X'04'
ORG PTABLE+C'A' X'C1'
ORG PTABLE+C'J' X'D1'
ORG PTABLE+C'0' X'F0'
The ORG , at the end instructs the Assembler to resume at the highest point. It might be safer to use ORG PTABLE+256, but it's just not done. This table tells TRT to stop if it encounters a code that is not A through R or 0 through 9.