I can't find a decoder for this VIN on my olsdmobile because it's 14 digits and I haven't found one for 14 digits. Look.
The number I see is: 3BN39LAN246431

 

This is the Fisher Body data plate. It only lists the series and related assembly info, but not the VIN.
The VIN should be visible in the bottom corner of the windshield when viewed from the outside of the car on the drivers side.

 

As oldoldss mentioned the cowl tag has limited info.

It can be somewhat difficult to see the vin number itself but it is where he stated. It is a fraction of the cowl tags size.

Here is a similar cowl tag. Pic #52 https://bringatrailer.com/listing/1973-oldsmobile-88-4/

And its vin number is pic #53. (As you can see with a black dash it gets somewhat washed out. Till you get very close.)

Once you locate your vin number this decoder will do the trick. https://www.v8cars.hu/oldsvin/decode.php

 

The cowl tag isn't the VIN. SOME of the data on the cowl tag will correspond to the VIN, but most does not. Most importantly, the 246431 BDY number is the Fisher Body sequence number for the body only and is unrelated to the VIN. If you were to have the build sheet, this sequence number would be tied to the VIN on that document. From this cowl tag, we know that the VIN will be in the form of 3N39x3Mxxxxxx.

3 in the first position indicates the Oldsmobile Division
N in the second position indicates Delta 88 Royale and corresponds to the BN on the cowl tag (the B indicates that this is a B-body platform)
39 indicates the four door hardtop body style
The fifth position will be an engine code letter. This is not on the cowl tag since Fisher didn't install the engine. The only options in a 1973 Delta 88 would be N for the L65 350 2bbl motor or T for the L74 455 4bbl motor.
3 in the sixth position indicates the 1973 model year, as shown by the ST 73 on the cowl tag
M in the seventh position indicates the Lansing assembly plant, as shown by LAN on the cowl tag
The final six digits are the assembly sequence number.