I have been trying to find this since this post started, finally found it right where I put it, just couldn't remember where that was! crazy It's an original advertising flier that measures 3.5" x 6". (PS - yes I did find it in the last place I looked).
[Linked Image]

I looked at the serial numbers that have been documented so far and took note of the gaps between them. The 4 largest are 687, 537, 347 and 292 - the smallest 15, 16, 17 and then about 25 (is from incomplete serial no.). The gaps seem to be evenly spread out with the 2 of the 4 largest coming from the ends. From this I would not expect to find a larger gap on either end of the numbers collected so I do not expect a no. 1 and would think it reasonable that 1000 may have been the start, I also would not expect to break 6000 (or not to go over by much). I have looked for several different model guns over the last few years with known production numbers and really feel from what I have seen that 1000 is to small a number for the 425, I would be more inclined to think that we are looking at about 5000 that run from 1000 to about 6000.