"Messenger M4V" for those that might know ?

Friend of mine passed away about a month ago , (May he rest in peace) got a call from his executor and my friend had left me his Messenger M4V (1 MRF 455 driving 8 x MRF 455s) . He had it in one of his cars over 25 years ago and not for very long . Well kept in a box for all those years . Excellent condition next to mint . I hooked up everything and it worked flawlessly using a Grant XL 1 watt swinging about 15 AM and 15 on SSB , on High was seeing pretty much 125 dk swing 500 all day and it was the same on SSB , (meters very) everything seems nice and smooth and SWR's are very nice . I have a few questions about it that I couldn't find on line, there's a rocker switch on the back that reads 4 watt and 10 watt not knowing I just keep it on 4 watts and do what I'm doing with it now. The other question would be about age of the units ? This one has February 23 , 1993 engraved on the heat sink . So I'm guessing this is a later model ? When did they first come out and when did they quit making them ? My friend had a museum of radios and amps of all varieties. You could pretty much say it was a Bill Good museum , they were friends and Bill did all his work . Thanks for any help that could come my way on this one . Switch Kit

Thanks Bob and Jeff for your input on this one. I never really heard to much bad about them over the decades , I've heard something about thermal tracking but that's way above my pay grade ? I've seen nice ones on Ebay fetch upwards to a 500 and a thousand dollars . (Crazy prices these days !) really nice of my friend to leave it to me , just wish he was still here to hear it. It's kind of strange how I feel about it for now , like maybe it would be safer back in the box . I've never really been much of a collector over the years other then just moving equipment along as they came along. I used to know some people who could have answered my questions but unfortunately they are no longer with us and may they all rest in peace . Thanks again . Switch