I'm not sure I'm willing to conclude any particular meaning to the FD3 vs. FD4. In particular, I'm not convinced that they tell us anything about when the parts were made. My theory would be that FD designates the supplier that made the rear-leg castings. Black & Decker didn't do any casting themselves; they used numerous outside suppliers simultaneously. The FD3 and FD4 would be two different molds the supplier used. I've also seen an FD2, leading me to think there was probably also an FD1. It's possible the FD3's preceded the FD4's, but I would expect they were using both molds at the same time. If you examine the other cast parts you'll see other codes; I remember an S in a circle and a BM, which I think would be examples of other suppliers casting other components. I also think the 72 or 74 is the date the part was cast, not when it was assembled into a Workmate. Depending on just how they managed the parts when they arrived at Black & Decker, those could be very different.
I'll also point out that In addition, as far as I can tell, the same rear-leg casting design was used throughout the entire production run of the WM325 Type 1 through Type 3 (which of course also included the 79-001 Type E), whereas the galvanized step and its tab changed at some unknown time. There's no reason I would expect the FD3 vs. FD4 castings to match up with a difference in the tab on the step.
When we discussed this in 2023, I compiled a list of these characteristics from examples of WM325's and 79-001 Type E's for which I had photos that showed enough info, shown below. It's fairly limited data, but I couldn't see any pattern in it. For example, I found only two Type E's with the long tab, one with an FD3 rear leg and the other with an FD4. With no apparent pattern, I did not continue to try to collect this data after that one compilation.
First, the WM325's:
Type|Mold|Year|Tab length
1 ? 72 NA
1 FD3 74 ?
3 FD2 72 NA
3 FD3 74 long
3 FD3 74 long
? ? 72 ?
? FD3 74 short
Then Type E's
Type|Mold|Year|Tab length
E FD3 74 long
E FD3 74 short
E FD3 74 short
E FD4 74 long
E FD4 74 short
E FD4 74 short
E FD4 74 short
E FD4 74 ?
E FD4 74 ?