Metropoulos Forum

Anyone? They both test in the low "good" range, but I only paid $2 for the two

Yes, RCAs are famous for their "stop sign" labelling. These two are just from different production runs whose construction tended to vary over the years. One on right is true 50s blackplate, long covetted by audiophiles and the other one on left is a grey plate from the 60s. Both are excellent hi-fi tubes, but I've never tried the NOS grey ones I own in my Marshalls because I'm saving them for my home audio stuff. I'd expect a somewhat mid-scoop in a Marshall with these like I've noticed with Teles. Their frequency response imparts little colouration to the signal, unlike Mullards that have a nice enhanced mid. Of course, depending on the voicing of YOUR amp, I could be completely wrong!

I'm 99.99% sure that those were made in the same plant as GE and Sylvania tubes. I have a Tung Sol that has that same "boxed in" tube designation and I believe it was made in that factory as well...

They're both blackplates, actually... the one on the right looks grey in the pic. Left tube has shiny plates, right has dull plates.

I've been told the one on the left is likely a Raytheon, the right an RCA.

I not for certain, but I don't think Raytheon was every a manufacturer, just a recipient of re-labelled tubes.