Speedmaster Professional 3590.50 or 3570.50 ? Some sort of transitional?

Hello all.

Some help / advice please.

I have a Speedmaster “ Moonwatch” Professional, c. 1996.

Serial No. is 4835**** It has the Tritium dial. The movement is stamped “Cal. 861” and is predominantly a gold colour. It is also stamped / engraved as having 18 Jewels.

Movement serial number and case numbers match. The inner case back is stamped 345/0022 and 145/0022

As a bonus ( to me anyway ! ), it has its original tapered 1499/842 strap which, I understand

was only supplied for a very limited time.

So, that’s a description of the watch detail. I’ve attempted to attach a couple of pictures or so, but the photo box is asking for a URL. The pictures are actually on my computer and won't copy over.

So, myy questions are: Having a Tritium dial, Cal.861 movement but with 18 jewels, is this some sort of a transitional watch falling between the 3590.50 and 3570.50?

If so, which model is it?

In my efforts to find some information / illustrations on 18 jewel Cal. 861 movements, I can only find Cal 1861 as having 18 jewels. So, do we know how many Cal.861’s there are with 18 jewels and Tritium dials?

Just want to pin this down if you understand. Any help or advice would be much appreciated. Thank you.