I have owned a lot of Mopars in the last 30 years or so and have come across a car one of my relatives that has passed has left me.
They were the original owners and the car only has 40 k on the obdometer.
I am trying to find out why this car has a 273 2bbl that did not seem to be offered as an option. I have had several other enthusiasts inform me that there is no way this combination was available as this was the first year the 273 was offerd and it was mostly put in Darts. Without a serial number to confirm this on the engine block and the build sheet being lost when the back seat was redone back in the 70s because of a cigarette being thrown out the window and ending up burning a huge hole in the seat. Is there any other way to assure my fellow enthusiasts that this weird combo is indeed from the factory?

Thanks in advance!

I'm gonna say someone put that engine in that car for whatever reason. Dealer/mechanic yes, factory, prob not...
Once, I bought a car from a relative, a 71 Monte Carlo. Went to auto parts and bought all the standard tuneup parts. Points, plugs, cap, etc. Drive to work next day and started to run rough, barely made it home with one heck of a noise under hood. Two plugs blew out of heads, couple more were loose. Ends up that the plugs I put in were tapered style, but the old ones had washers. 71 first year for tapered plugs. Eventually found out my long block had been replaced for a mechanical issue. They thought the dealer just fixed the original engine when I talked to them. They weren't sure what was the problem originally, but dealer did fix it and it was fine afterwards.

The 273 was Dart/Valiant only in 64,nothing else.

is there a fender tag on the car?

what's the VIN & fender tag info?

I have seen it many times, the poly blows and someone buys a "318" to put back in it, but it looks "different". The trans bolts to it, and everything seems to hook-up fine, must be the right engine. Sometimes a later 318 car has a 273 installed because that is all the mechanic could find at the time. Unless you owned it from new then you simply do not know what happened along the way.

The car was owned by a relative and purchased from the dealership new and they had all of the records for service since new. No way to actually prove if this 1964 273 came in the vehicle? The VIN or the Fender tag do not show the motor info from what I can tell looking on the standard sites.
You positive there is no way this is the engine that came from the factory?
I've asked a few of the elder relatives and they are sure the way Gramps took care of the car that if the motor had blown he would have been devastated and they would have heard all about it.
The car is pretty nice, the old guy took care of it.
I'm going to have to change my vehicle discriptions if I can't find a way to support the numbers matching claim.
I had n idea this was such an odd ball set up.

if you are decoding the fender tag properly? is there a broadcast sheet?

you could also request an IBM decode from FCA Historical & see the original selling dealership & option info, to compare with family recollections

how late in the model year was the car built?
there should be some supporting evidence on/with the car that would verify whether or not the engine has been changed: parts, date codes, castings, etc.

Numbers matching didn't start until 1968, so before that there is no such thing. Look at the casting date on the side of the block.

Post a pic of the fender tag...

The 64 Polara came with either a slant 6, a 318 poly, a 361 2 bbl, or a 383. The 273 came as a mid-year option on the Dart, Valiant, and Barracuda.

& 360s were introduced on a limited number of '70 vehicles
& 400s were introduced on a limited number of '71 vehicles

so, without more information/documentation & follow-up by the OP "WE" can't help

Don't forget max wedge and street wedge.