In the mid seventies, probably in 1975, perhaps in 1974, definitely by 1976, the production of Lambert bicycles was taken over by an old established English bike manufacturer by the name of Trusty. Jigs for the fillet brazed frames were transferred and the stocks of Lambert branded and special components. The bicycles were now made in Birmingham, England and were called Viscount Aerospace bicycles. Decals / stickers on the down tubes noted that the frames were made by Trusty.
The first Viscounts, as might be expected, were very similar to the Lamberts but, over the next few years, they evolved into rather different bikes. Keeping to the same Aerospace fillet brazed frames they sported more modern paint schemes and over time most of the specially-made components disappeared in favour of bits from Shimano and other component makers.
Building on the experience of Trusty, lower-range bike were also badged as Viscounts. The chronology of this is unclear, but I think that they came in before the company was taken over again, this time by Yamaha, probably in 1978. They included ones with lugged frames that were built in the UK and ones using Taiwanese-made lugged frames. Of the first type were the Sebring 10-speed model, which seems to have been a very popular entry-level ‘racing’ bike and one that many people remember with affection. It began the use of US cities as the model names of Viscount bikes. I had a Tulsa G.L., which had a made-in-Taiwan frame. A collection of details of all these models needs to be made.
In 1980 or ‘81 Yamaha-Viscount stopped using the fillet-brazed frame. The top-of-the range bikes still used the same tubing (or appeared to) and were called Aerospace but the frame was built with rather natty pointed lugs with windows (Prugnat?). Production had at some point moved from Birmingham to Potters Bar, North London (Hertfordshire), where Trusty had a plant. By 1980 use of special, own branded components had entirely ceased.
I have a 1981 catalogue in which the top of the range is one with a Shimano Dura Ace group set. I’ve never seen one of these. Second was probably the Viscount Aerospace 600ax, with the Shimano 600ax gruppo. Just below that must have been the 600ex (you can guess what that came with). I have one of these and gorgeous it is too.
Then there were a number of variations around the Viscount Aerospace 400 theme. It may be that they were intended to use the Shimano 400 components, but my 400 came with a 600 rear derailleur, Dura Ace front mech, Weinmann brakes and SR chainset. Incidentally Zeus Olimpic 64 pedals were used on lots of Viscount Aerospace bikes, irrespective of the other bits used. But I am digressing into components.
Despite successes in various races and being named as bicycle of the year in 1980, the marque seems to have disappeared completely by 1982 or 1983.
There is a group on Flickr for photographs of Lambert, Viscount and Trusty bikes.