I suggest you contact Omron with your requests for Marshall 80-series blood pressure monitors. Omron seem to have bought / acquired / become otherwise responsible for the Marshall brand.
That is how I got the manual for model 80 here
Owners Manual User Guide for Marshall Professional Aneroid Sphygmomanometer...
To request a manual from them send a message to them with your exct and detailed request using the webpage
Contact OMRON OMRON Global
I hope this helps
24.February.2015