For a few seasons, he was inarguably a great catcher.

exactly. I think what needs to be asked here, is how much value can be given to his years playing in the California league.

sub-questions: Was he playing there because he could no longer hack it in the bigs?
answer: I doubt that. He had a poor season in his last year in the NL with Pittsburgh (OPS+ of 84) He apparently had a hand injury. The Pirates let him go. He was given major league money to play out west in an emerging league. Bearing in mind that back in 1891, there was not quite the same distinction in terms of prestige between leagues, the differences being in financial terms. This was before the reserve clause. Players went with the money. The money, for Carroll was out West. So was his home. Seems a natural fit.
Bill James takes as his source for Carroll, William E. McMahon's SABR article from Baseball's First Stars.
First year out west, Carroll led the league in both average and HR. The second year, on a different team, he hit .338.
1894 (Western League) - .389, 51 doubles, 23 triples, 22 homers. 223 hits and 186 runs in 130 games.

1895 - .414, 58 doubles, 21 HR, 216 hits in 122 games.

And that was it.

So how much credit can be applied to that?
Somewhere safely between none and full. I wouldn't go so far as to say that the level was as good as the NL, but it wasn't Little League, either. It's fair to say that he didn't go there because he couldn't play in the NL, but he didn't want to.

Until I can get a better understanding of what those seasons really translate to, I am awarding Carroll enough merit to give him 2 NL seasons with OPS+ in the range of 110.

That puts Carroll 4th in my Catcher backlog.

1894 (Western League) - .389, 51 doubles, 23 triples, 22 homers. 223 hits and 186 runs in 130 games.
1895 - .414, 58 doubles, 21 HR, 216 hits in 122 games.

two seasons in Ban Johnson's Western League?
That would be the midwest today, something like Omaha to Detroit (gradually moving East, including Columbus and Buffalo in 1899).

I weould suggest that, like Fred Dunlap in the UA in 1884 or ML players who were in the service in '43 and '44, that you just give Carroll credit that is more or less a straight line projection from his ML value over the last 3-4 years of his career (preferably the previous 2 years, but then with consideration given to the fact that he was injured in that final year).

I mean, those numbers for '94 and '95 are nice.

And it sounds like that's basically what you've done.

I could see Carroll 4th behind Bresnahan, Trouppe and Mackey, though maybe behind Lombardi, too, and then for me there's Clapp.

sunny - that's pretty much how I have him. My only difference is that I switched Mackey for Lombardi - Carroll between them.

.
According to William E. McMahon, "Frederick Herbert Carroll" in Baseball's First Stars [19c Stars, II]:

Fred Carroll "was known primarily as the battery mate of Ed 'Cannonball' Morris, with whom he went East to play with the Reading Actives in 1883, after which they played together until 1890." Carroll played in the majors 1891, one year longer than Morris.

1892 - captain, Oakland, $225/month
1893 - San Francisco
1894 & 1895, Grand Rapids and Grand Rapids/Kansas City - "two monster years in the Western League"

that's all he wrote?

that's all he wrote?

No, there was more, but Paul wasn't going to post the whole article. :-)

Based on this information, I'm willing to offer credit. I gave him 18 (the average of the previous three years), 12, and 6 WS in declining order (willing to rethink that of course if someone has a better scheme). OK, so that gets Carroll to roughly Darren Daulton/Jorge Posada territory in terms of career value, but it doesn't change his peak. Among eligibles, he's behind Clements, Kling, and Clapp.

An obvious case of bias in favor of catchers whose names start with a "cl" sound. What if we agree to call him Fred Clairol.

Carroll was only my radar for a while, but I never voted for him primarily because his defense rated as very poor, and catcher defense was very important back in the 1880's...