In 1966, the Imperial Typewriter Company Ltd. became a wholly owned subsidiary company of Litton Industries.  Litton already owned Royal-McBee (bought in 1965 and merged into Litton Business Systems in 1967), one of the largest typewriter manufacturers in the world, and in 1968 would also buy out Triumph-Adler.  This huge conglomeration is remembered today for many things, but perhaps can best be thought of as one which mixed around various typewriter designs under various brandings, confusing matters for historians today.  Further confusion comes from the fact that some makes and models were actually machines not produced by any company owned by Litton, but rather were subcontracted.

In the case of Imperial, we can find three owner's manuals in my collection that help show the transition.  Illustrations from each, with comments, are provided.
Here we see the Imperial 70, introduced in 1962 as what would be the last version of the Imperial standard while Imperial remained in its original ownership.  The machine featured not only a removable platen, but a removable carriage as well.  Views from the instructions show the general plan and the platen removal.
The Imperial 80, introduced 1968, was the first new model introduced after the Litton buyout, and the first Imperial to include the "li" emblem that was the trademark of Litton.  While some features were altered, the basic mechanisms were the same.
Serial number records indicate two different series of Imperial 80 running through 1971 or so, and then end.  Our evidence as to what happened next is seen here.  This is a scan from the instructions for the ROYAL 480, which were printed in England in February 1974.  This is the same machine as the Imperial 80, but we note the use of the Royal and not the Imperial name.

This machine was available at the same time as the Royal 470, which despite the closeness of numbers was completely different, and was developed from the Royal standard and not the Imperial.  (It appears as if the Royal 470 was also built in England, probably at the Imperial plant in Hull.)
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The Imperial plant in Leicester was shut down in 1974, and production of Imperial-pattern standard typewriters ended.  Litton was, around this time, generally attempting to close plants in the US and England (it had closed Royal's plant for portables in Springfield, Missouri in 1970 and soon would shut down Royal's original massive Hartford, Connecticut standard typewriter plant) and move production to other countries such as Germany, Japan, and later Portugal.  The Imperial (and Royal) names stayed around much longer, though, and so did the sales networks. 
At left, a lucky find.  This manual was sent to me in a block of many, and the former (typewriter repairman) owner has clearly indicated that this manual, while written for the ADLER Universal 200 / 390 / 400, also applies to the Imperial 90.  The date of the manual is 12/74; luckily, we have a number of previous printings of this manual and note that this one is also the first to mention (on its rear) the T.A. Vertriebs-GmbH amalgamation of Triumph-Adler, and is the first to omit the name "Adlerwerke."  It seems clear then that the general melting pot of Litton's typewriter production of the 1970's included at least one Adler-pattern machine built in West Germany which was sold as an Imperial.