Looked at model 13-4 today a little high at $399 looked pretty good had a strange thumb piece old style with the bottom half gone? Looked in the s&w "bible" didnt see such a thumb piece. Have a 64 and 686 so whats a good reason for this piece other than another toy.

Take it easy.

You are not giving us much to work with here. There are some M13-4 units from the Performance Center that have a thumbpiece like you describe, these are the Model 13 Quad Port guns. If that's what you are looking at, then get in your fastest car and go buy that gun for $399, it's worth double that (easily IMO).
On the other hand, if you are looking at a standard gun with a modified thumb piece (or one replaced with the PC version), that's a different story.
More data, blue or nickel, barrel length, ported or not, round butt or square, standard hammer or bobbed?

Here are the M13 Quadport guns mentioned above, note the thumbpiece that the PC uses:

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.............SmithNut

Thats the thumb piece alright but not the same gun its a blued sq butt 4" heavy barrel fixed sights magna grips and sa/da hammer.

Take it easy.

The condition and to some degree your geographic location. While I've not been following the M13's of late, I'd have to say that the price of $400 is on the high side to a minor degree, others might disagree.

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.............SmithNut

That PC 13 is a damn' fine looking revolver.

The model 13 in 3" is nice because of its uniqueness. Generally speaking, in a 4" I would rather have a model 19 with target grips, but that's just my personal preference. There is little doubt that model 13's in mint condition are much more rare nowadays. That hasn't necessarily translated into a higher price for the 4" at least. I have seen mint condition model 13 4" in the price range slightly above $400 with box and whatnot. Now, if I found a mint 3", I'd jump all over it.

where can i get that underlug 13 barrel in blue without ports? i have a ladysmith 65, but in matte blue with bob hammer DAO that woud be the perfect carry gun.

cool PC gun.

Unfortunately they never produced any to my knowledge.
One would think that some barrels were made and never made it to the porting step, but I've not seen or heard of any.....

Bummer

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.............SmithNut