Yep, Long & Marshall. Engineering company that also made RC car bits. They were near to where my parents live, used to get cranks, rings etc from them.
The earlier versions had a webbed structure on the back of the arms. This tended to crack so as machining techniques improved they moved to a channel. DH ones were solid.
L + M were NOT german.., very much uk.
one of the owners was Peter Lane (who was the L in L+M) i know this as i used to work with him and sell the cranks through the uk company zoobits.
hope this helps