Some of the early PAL chips had no security bit to blow.

These are the earliest programmable PALs. Because the security fuse feature did not exist yet, these chips can always be read back using a legacy device programmer (assuming the chip isn't broken). They have a standard 32x64 or variant fuse array but no "extra" security fuses at the end of the line. [1, 2, 3, 4]

The Original and A-versions of these chips had no security fuse:

• PAL10H8 and PAL10L8
• PAL12H6 and PAL12L6
• PAL14H4 and PAL14L4
• PAL16H2 and PAL16L2
• PAL16L8 and PAL16L8A
• PAL16R4 and PAL16R4A
• PAL16R6 and PAL16R6A
• PAL16R8 and PAL16R8A

These chips have security fuses:

• All "B" and "D" High-Speed Series 20 variants (e.g., PAL16L8B, PAL16R8D)
• All Series 24 Extended PALs (e.g., PAL20L10, PAL20R8, PAL20X10)
• All MMI MegaPALs (Large high-density legacy chips like the PAL32R16 or PAL64R32)1]

One of my ex bosses explained why he never bothered with the security fuse. If people are sufficiently determined, they will reverse engineer combinatorial PALs almost instantly, and registered PALs with a bit more effort. If a competitor started selling copies of our product, they'd simply get sued until they stop.