Unidentified Part

[Ben sage]

I don’t know what this is it looks like a old transistor much appreciated if anyone knows

Thank you

[Whales]

Could also be a rectifier sandwich?  Odd to have 3 legs,  but possible.

What's the nearby circuitry?  Mains power input?  RF?  Audio?

[Doctorandus_P]

It's a grey thing with red wire around it 

A quick search pops up that bradley labs (probably used to) make selenium rectifiers, in which case this could be a dual diode.

https://duckduckgo.com/?q=%22bradley+labs%22+III+C+electronic+component

[fcb]

https://books.google.co.uk/books?id=lk5MZWfJRqAC&pg=RA1-PA1&lpg=RA1-PA1&dq=bradley+labs+inc+components&source=bl&ots=tnsY7fkdlR&sig=ACfU3U16rloOgZylWqlLssbLviv06rMbwQ&hl=en&sa=X&ved=2ahUKEwj6yKGNo6XoAhWSolwKHRt1BPQQ6AEwD3oECAYQAQ#v=onepage&q=bradley%20labs%20inc%20components&f=false

Selenium (arc) rectifier??

[TheMG]

The sandwich construction leads me to believe possibly a selenium rectifier (a very low current one due to absence of plates to dissipate heat).

Note that selenium rectifiers have a high forward voltage drop compared to silicon diodes, around 1V per junction, and due to their poor reverse breakdown voltage (around 25V) they usually stacked several selenium junctions together to increase the reverse breakdown, as such most selenium rectifier can not be measured using the diode test function of a standard DMM since the total voltage drop is several volts. This also has some possible implications if replacing the selenium rectifier with a modern silicon diode.

Also note that selenium rectifiers often go bad after prolonged periods in storage, so don't necessarily expect to work at all. I found a bunch of NOS ones at work in the parts bins a while back, and they were all open circuit (tested using Huntron Tracker).

[tkamiya]

Can you take a photo from back side?  (the other side of labeling)

To me, it does not look like a selenium rectifier.  Ones I've seen all looked like fins spaced apart, not too unlike part of a heat sink.  You could remove the unit, noting how it was assembled first.  Measure between each pins and see.

[mzacharias]

The sandwich construction leads me to believe possibly a selenium rectifier (a very low current one due to absence of plates to dissipate heat).

Note that selenium rectifiers have a high forward voltage drop compared to silicon diodes, around 1V per junction, and due to their poor reverse breakdown voltage (around 25V) they usually stacked several selenium junctions together to increase the reverse breakdown, as such most selenium rectifier can not be measured using the diode test function of a standard DMM since the total voltage drop is several volts. This also has some possible implications if replacing the selenium rectifier with a modern silicon diode.

Also note that selenium rectifiers often go bad after prolonged periods in storage, so don't necessarily expect to work at all. I found a bunch of NOS ones at work in the parts bins a while back, and they were all open circuit (tested using Huntron Tracker).

I'm not sure you could see the "junction" with the tracker due to a much larger forward voltage drop; or does it feature a specific test for selenium sections?

[ArthurDent]

Generally selenium rectifiers that size were used in VOMs as meter rectifiers. The selenium disks are pinched under quite a bit of pressure in the c-clamp type metal holder then painted to keep moisture out and keep the assembly stable. Here's another one that was used by Simson on one of their VOMs. The one in this photo I found on line is a full wave bridge so it has 4 leads

[TheMG]

I'm not sure you could see the "junction" with the tracker due to a much larger forward voltage drop; or does it feature a specific test for selenium sections?

It's a Huntron Tracker HTR1005, goes up to 120Vpp on the high range, so there should be no problem at all!

To make sure I even hooked up one of the rectifiers to a bench power supply in the forward direction and cranked the sucker up to 60V.... absolutely nothing.