Speaking of Giants catchers, IMO the true Moby-Dick in Thursday's group was Jerald "Jake" Brown, who caught in college and the minors, but not during his 48 PA in the bigs in 1975.
Born in Sumrall, MS but raised in Houston, Brown spurned a 33rd-round drafting by Twins in 1967 to attend Southern University and was rewarded two years later when the Giants made him the 2nd overall pick in the June 1969 Supplementary draft. (Rich Hand was #1.) Five seasons in the minors followed, culminating in batting .289 and .290 for AAA Phoenix in 1972/1973, which put him in San Francisco's plans, as Mike Sadek had hit a Sad .167 backing up Dave Rader in 1973.
Unfortunately, just two weeks before he was to report to spring training, Brown broke his left arm working at his off-season job (a steel mill in Houston), and was out for the year. He never caught professionally again.
Still, this didn't prevent the Giants from at least posing Brown with a mitt the following spring:

(As is often the case, I've tweaked the brightness/sharpness to get the face out of the shadows.)
Brown was sent to AA, possibly out of concern for the missing year, or by request, as the AA team had just moved to Houston-convenient Lafayette LA. He wasn't there long, though; a month of .307/349/453 and after the Giants dealt off Garry Maddox to the Phillies on May 4th, they needed another outfielder and soon after, Brown got the call to the big team.

His first three appearances were as a pinch-hitter, and he did fine, collecting a double and a walk. This earned Brown a pair of utility starts on May 27/28th in Philadelphia, replacing RF Bobby Murcer on the 27th and LF Gary Matthews on the 28th. Both games saw Brown go 1-for-4.
After an off-day to travel to Montreal, a rain-out, and a game where he only pinch-ran, Brown got what looked to be his big break on Sunday, June 1st, when Matthews injured himself in the pre-game warm-up, breaking his thumb and spending the next 6+ weeks on the shelf. Brown was inserted into Matthews's LF spot (and clean-up spot in the batting order) and in the top of the 1st he found himself facing Expos starter Dave "Maybe I Should Just Quit and Go Home to Montana" McNally (making what would prove to be his penultimate start of his career) with the bases loaded, and Jake doubled to score Gary Thomasson, Bruce Miller, and Murcer, giving the Giants a 3-0 lead. Things looked good.
But…in the bottom of the 3rd, Larry Parrish started the Expos' comeback, homering off of Mike Caldwell. It was a windy day at Parc Jarry (the Expos would hit four dingers, and once McNally [who'd always flustered him] was gone for the day, Murcer would hit a pair of 2-run shots to get the 13-5 win for SF) and I guess Jake didn't think Parrish's ball was headed out, because he went back to play it, ran into the wall, and was lucky to escape with only a severe right-cheek contusion. But he was out of action for the next 12 days, and missed his chance to lock down the LF job while Matthews was out.
By the time Brown returned with a pinch-hit double on June 13, Westrum was committed to playing Gary Thomasson, no matter how low his average might sink (as low as .156 at one point) and despite the annual "June swoon" the Giants were experiencing at this point. It was Glenn Adams in LF, Thomasson in CF, and Murcer in RF, and when CF Von Joshua got off the disabled list, Thomasson was shifted to LF, while Brown stayed anchored to the bench. He got one start against the Padres in SF (went 0-for-4) and the next week got to play both ends of a double-header in San Diego, going 2-for-4 in the opener but 0-for-2 in the nightcap, and that was the end of his starting opportunities. (In fairness, Brent Strom threw a 2-hitter in the second game of that doubleheader, and Brown did get one of the 2 walks Strom allowed.)
For the rest of the season, Brown was only a pinch-hitter or other sub, usually playing in lost causes (Giants were 9-32 in games he appeared in). His early success coming off the bench proved to be a mirage (his last hit was the 1st week of August) and his stats went down the drain. The Giants sent him back to Lafayette to start 1976 (again, Lafayette over Phoenix might have been a personal preference) and then included him as part of the 4-player package sent to the Braves in the Darrell Evans trade. He finished the season with the Braves' AA team in Savannah and then was released or retired.
Tragically, Brown died at age 33 in Houston of leukemia. Which leads me to wonder…was he gay? Young single men dying of "leukemia" in 1981…it wouldn't be impossible. (Yes, most of the early AIDS deaths were in NY, LA, and SF ["coincidentally" the same three cities where the US government was "immunizing" gay men against Hepatitis-B in the late 1970s…], but having played in San Francisco, Brown might have travelled back there later on.) Was
this the reason why Brown didn't seem to get a full chance, why Westrum was so set on keeping Thomasson in the lineup and even playing utility infielders like Chris Arnold and Steve Ontiveros in the outfield, while leaving Brown on the bench?
Maybe…but perhaps not. Brown could simply have had a lot of bad luck, from the broken arm at the wrong time, to the cheek contusion at the wrong time, to Westrum deciding that Gary Thomasson (final stats for 1975, .227/304/347) just
had to be in the lineup every day, even after Matthews returned. And you can fatally compromise your immune system in other ways, such as intravenous drug use (it was the 1970s, after all) or simply extreme poverty (Brown played before the "one day's service gets you on the pension plan" revision of 1979; he could have fallen to homelessness and contracted leukemia that way). Or perhaps he was simply born with inadequate white blood cells and was lucky to make it to 33.
We'll probably never know. RIP, Jake.