I picked up a nice condition Heisser back in April. This appears to be a plain Jane holster like the first on the chart listed above. The No. on the back is 210. Any idea what fits in it?

I don't presently have photo capabilities, old flip phone, but the monogram on back is an oval, JH. H. Heiser, Maker (in middle) and Denver, Colo.
Both brown snaps say Heiser Denver.
Will this help in the identification?
That's not a JH but rather a stylized H: Hermann Hugo Heiser.
Translation for my friend turnerriver: he thinks he has told the OP, "Yes, the holster is for a 4-3/4" to 5-1/2" SAA".
The three-digit numbering on Heisers is from their 'original' (that is, before Keyston) era and is the model number vs the fitment; so that one is perhaps more valuable. Agreed, pictures always clear up this kind of misunderstanding immediately; otherwise it's unpleasant guesswork.
View attachment 408089
NOW he can tell you what it is.
The so-called 'football' logo of Heiser's was used the most of the half-dozen marks for Heiser, known to have appeared by 1930 and to have been used right the way through the end of Heiser while owned by Keyston. The brown snaps are considered to have appeared around the time of the Great Depression; likely your rivet heads are plain vs marked HHH. I suppose I don't know if the brown snaps were used through to the end! But I'd still expect 1930s/40s. Turnerriver will know and surely will reply to a PM you send him.
Some pics of the items you've mentioned:
View attachment 408118
View attachment 408119 That's not a JH but rather a stylized H: Hermann Hugo Heiser.
No pics, no definitive answer; I'm unable to find any Heiser holster numbered 210.
I provided the chart for the OP to find the information he sought and for others to have the resource going forward, no translation needed, I thought.
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Regards,
turnerriver