Came into this clean 64 no dash 4"pencil barrel with a round butt.  reading indicates they were only available with a square butt in the 4".   Anyone ever heard of one like this?   D serial # prefix and the standard catalogue shows manufacture range of 1970-72.

Looks like a gunsmith special than anything but yeah I've never seen one like that.

Maybe a special order or gunsmith modified?  I never saw a 4" K-frame S&W of that era with a round butt, but I've read about some Model 19s.

With S&W, never say never.  I have an uncommon 4" Model 14.

They made quite a few 4” Model 10s with round butts, wouldn’t surprise me in the least if it’s factory.

They didn't make those from the factory. I asked in the pic thread but i guess you answered that question here.

I did mine
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Better pic
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A pencil barrel 64 is one that would be neat to own, but I don't recall any round butts offhand.  Likely a smithy special, like the polish job.

In 1992 I sent a 66 to Smith to time it . They quoted 191 iirc , then I got a letter saying it needed a new cylinder.
Then they wrote and said we can not use the frame either , out of spec .....

Then they told me to pick any gun they make , I thought about it for a day and told them I wasnted a 6" barrel , with round butt and polished to a nickle like finish .

A month later it shows up , they knocked it out of the park !

And they charged me the origanal price 191 bucks
This is when I put the square target grips they work with just a little gap .

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Never recall seeing one, so I looked in the 3rd Edition standard catalog of S&W and the only factory standard product round butt 4" 64 listed were heavy barrel versions (102506 & 162506), none listed with the tapered barrel that had a standard product code.  Most likely gunsmith work as cited above, either a barrel swap or rounded the butt.

I collect Model Tens and their variations.  I have never seen a round butt pencil barrel 64.  Pencil 64s in general aren't real common, far more heavy barrels.  I suspect custom job.  The polishing looks much better than most 64s as well, which also points in the custom direction.  Round butt pencil Model Tens aren't too rare, the military bought thousands of them in the Viet Nam war era.  I found one a couple years ago (not a military issue).  I prefer the RB versions, fits my smallish hands better.

Nice find.  I  almost certainly would have snagged it had I run across it.

But also, never say never with Smith.  There's a lot of uncatalogued weird stuff out there.  An RB 64 would have been easy to make.

So I have an update:
I looked over the gun in detail.

The serial on the yoke has no prefix (assumed to be "D") and is 5 digits.
Frame is marked "MOD. 64" under the yoke.
Frame is marked under the grips with a dot matrix 5 digit serial that matches the yoke.  
Those serials match the "pencil barrel" date range 69-70.
The bottom of the grip frame has a D prefix serial number in the 400,000 range that dates to the 1972-73 "round butt" range.   That serial is on the other side of the grip frame.
The cylinder is marked with a "V" on the rear face.

Maybe a frame swap?  $100 for a factory letter is probably not worth it.

That would sound to me more like a barrel/cylinder/yoke swap, as the official serial number should be on the bottom of the grip frame, not under the grips, and the dot matrix/electro pen serial matches the barrel/yoke/cylinder.

That, and the earlier parts are slapped on a later frame, instead of the other way around.

A factory letter would match frame/official serial number records, which it sounds like you dated to long after the pencil barrels were being produced.  The only way a factory letter would be worth the extra $100, IMO, would be if you think the barrel/yoke/cylinder were swapped onto the newer frame as a replacement for a worn out old frame (hence the pre-existing serial number), which would be kinda neat.

Still a cool setup.