Lots of poorly spelled details below!
EMI RS 61.
Possibly the best and most rare microphone preamplifier ever made on earth.In excelent working condition.Comes from EMI (Electric and Musical Industries), one of the top players
and inventor of the music inductry. Was used among others by the Beatles at Abbey Road Studios .Sends other great tube or not microphone preamps for tea. It was designed at EMI’s CRL Central Research Laboratory; one of the world’s top research laboratories. This is where many innovations took place and one of the key figures there was Allan Blumlein, after working at Western Electric International Labs, possibly the best Electronic Engineer of all times, best known as inventor of stereo record cutting . One of his 128 patents on simultaneus application of current and voltage feedback  to match impedances relates directly to this amplifier as both of these techniques are implemented if one follows physicaly the layout and interconnection of components. A D Blumlein died young in 1941 while radar prototyping testing. While at CRL he liked visiting the younger engineers labs educating them by helping them or sometimes was amused by playing music with his colleagues on test signal generators . While this particular amplifier was built on 1956, it seems to be one of his brainchildren and more of his patents made be related. As you can see from the serial number which is really a date code it was made on 1956 9th week or 9th month. This amplifier is designed to operate between impedances of 200 Ohms as this seems to be the protocol used by EMI. They had a switch to use if you do not want its input impedance to be 200 Ohms.
Cosmeticaly it is in good condition. It is in excelent working condition and has been cared of like a baby. Needs an external psu of 6.3V, HT, (not included). Buyer could even use the (LT, HT) option of an RCA BA6A limiter to supply a preamp. Has been tested with 6.3V ac, 160V dc. Comes with original female 4 pin connector to feed the LT 6.3V and HT dc. Everything is original. Comes with lid, not in good condition, but serves well its purpose of electromagletic
shielding and dust protection. Signal input and output is of course transformer balanced, by EMI transformers. Original balanced (3 pin) EMI connectors for input and output are included.
The capacitors used are excelent quality paper in oil ones, do not seem to be Western Elecric and not shure if EMI even made these, please do your own search. There are only two electrolytic capacitors as cathode bypass, so there is no reason why this microphone cannot survive with care a few centuries.This great preamplifier with perhaps the smoothest sound mankind had the chance to taste comes at the price of a small airplane. And with it you can fly on earth.