Viking 44 Deck Pump-out fitting

I know this will sound like a dumb question, but I'm trying to find a pump-out adapter, or more specifically, how do I find out what Viking used in the manufacture?
My marina store, most marine supply sites and Industrial Supply sources show 1-1/2 " threads and a 1-1/4 " threads, both with the cam/lock 11.5 top.
Neither of these fit. Seems like I have a 1-5/8 waste fitting.

1989 Viking 44ft Motor Yacht.
Thanks,
Mo

Thanks to all. I talked with Viking Service/Parts in Riviera Beach, FL and learned that while it is a 1-1/2 in fitting, it is NPTP, not NPT. Parallel threads or, as I know them "straight pipe" or ISP (international straight pipe). Since there is no taper, the plug looks larger then the end of the 1.1/2 NPT fitting.
Regardless.......there is an adapter available from Lewis Marine Supply and Lamb's Yacht Center will have it here tomorrow for me.
Another problem solved by stubborn perseverance (and research)
Now onward to the next boat caper...............

Glad to do that. After all, that's why we're all here on the forums.....

I made the rounds with this issue followed several leads, including one parts man who said that the part would be BST British Straight Thread.
It was not. I have purchased a couple of "sure thing" fittings which didn't work.
Viking tried but was actually no help. Their blueprints show a threaded waste cap assembly but those actual parts are supplied by various contractors, much like the deck hatches and cleats. You won't find answers there.
Bottom Line:
Wasting too much time trying to get a fitting when 99% of all pump-out stations use a rubber cone which seals the opening during pump out. In all honesty, I have not needed the fitting as mush as I thought.
I was about to find a guy with a lathe and make my own from a PVC fitting, but as I said above, it's really an overkill project because of the way most pump out systems work........
All the best to you.......
Bobby Mo