I don't know in what literature they mention a name change from M.s fulvoguttatus to klappenbachi?
The name changes i found in the descriptions of the species, and in places around the web, is that the M. stelzneri fulvoguttatus was concidered a subspecies of stelzneri but became a species on it's own, M.fulvoguttatus.
The other reviewed specimen was stelzneri stelzneri, i can't find the pdf on klappenbachi as quick as i found the description of the change from fulvoguttatus.
I'll post the link underneath for you to read.
Maybe then you'll see that there is a lot of wrong information around the web concerning this species, along with others.
Recently i found a renowned site showing wrong pictures with R.reinwardtii as well, with this toads, pictures are more off then right.
The real stelzneri is not from Paraguay, but from Argentinia and Brasil.
The differences between these 3 are easy to spot when you've read the descriptions and saw the holotypes.
I'll post some as well so you can see the differences.
Maybe you can compare them to the descriptions to see if i'm off on this one.
As soon as i find the pdf i'm looking for i'll send it to you.
Differences between klappenbachi and fulvoguttatus are:
M.klappenbachi: dorsal: large, irregular yellow spots, sometimes black in the yellow spots, yellow spots or blotches dorsally inter-ocular group which forms an irregular band, has a straighter head profile;
M.fulvoguttatus: dorsal: many small, regular yellow spots, not black in the yellow stains, no yellow spot or spots dorsally inter-ocular group which forms an irregular band, has a more rounded head profile
M.stelzneri shows most resemblance to klappenbachi.
Only there is no black in the yellow blotches, also the yellow "mustache" isn't found in the stelzneri but does occur in most of the klappenbachi.
Also the back of the stelzneri shows no spots or blotches.
Article about fulvoguttatus:
This are the pictures: