The fact that the led is on would indicate that the 20v at the power supply is Ok.  Howver, could still be a connector from the power supply to J107 - 6.

Having read the schematics in some more detail I am almost convinced that the problem has to be either the LS374 or the connection to J107-6 (20V line).

Here is what we know so far:

The problem is present on all flashlamps that run off J107-6. There is confirmed continuity between J107-6 and the flashlamps, and the 20V tests OK at TP7.

Now, I have to assume that there is a separate circuit for each of the flashlamps as each can be driven individually. However, all playfield flashers (8) could fit on to one LS374 (8 flip-flops) so I guess they all use the same LS374. There are also another 5 flashers driven off the 8 driver PCB and not the power driver PCB like the other 8. These 5 are the LS, RS, Special, Totem Multiball and Jackpot Multiplier flashers. If these don't work it has to point to J107-6.

While there is continuity between J107-6 and the flashlamps, it is possible that the connection is not great (yet still showing continuity) and is causing a large enough voltage or current drop that the flashers are not being driven.