Threaded Headset Help

[PWong87]

I'm thinking of replacing the headset on my Merckx and trying to figure out the stack height I'll need on the new headset.

I believe I have a Stronglight A9 (see attached pic) based on faded "Stronglight France" written on the locknut. Only problem is I'm measuring about 40mm of stack height but I think the A9 has a stack height of 38mm.

Anyone know why this might be? Did the previous owner swap out the washer for a spacer? I was hoping to get a CK Gripnut or Cane Creek 110 that only have stack heights of 38mm.

Btw I calculated my stack height by taking the locknut off I measured from the bottom of the crown race seat to the top of the steerer tube then subtracted the headtube length. I guess I could remove the whole headset but that's more involved.