The holster has obviously been wet-formed (either by hand or in a press), and the cylinder fluting appears correct for a 5-shot cylinder as the J-frames have. Wet-forming is done using either the actual handgun or a replica (typically cast resin or aluminum), and the result should be a close fit to the piece used to do the forming work.

That would be my question; what piece was used for the forming? A number of holster makers adopt a "one size fits several" approach, using one forming piece to make holsters that will accept several handguns of similar size and profile. That approach requires that the forming piece used be the largest in all dimensions of all the handguns in that range, so some will fit a bit more loosely than others.

Handguns similar in overall dimensions and profile to the S&W J-frames include the Colt D-frame revolvers, Ruger SP101, Taurus 85 series, some Rossi's, and probably a few others. While similar in dimensions and profile, none are exactly the same and some have larger dimensions than any of the others.

A holster closely formed for the J-frames will not accept the Colt, Taurus, or Ruger. But a holster closely formed for these others will usually accept a J-frame (although not with a perfect fit).

The "dummy guns" frequently used by holster makers are made by several companies. Some are better than others in details and dimensions.

I suspect that the maker's forming piece is not a really close copy of the S&W J-frames.