Part number 57161 is the part original to the 1970 that I own.
Part number 57162 is the stud mounted one.
The difference is explained in the link L.A.B. posted and is shown below. I cannot imagine the push in type could vibrate free but, apparently, it did enough to require production of the alternate reflector.http://www.britcycle.com/Products/Reflectors57161&57162.htmFrom the Britcycle website: "For under tank mounting on 1970 and earlier Triumph twins, the 391-57162 reflector and associated parts noted above can be used with reflector housing bracket 83-2903, replacing original housing 82-8194 and push fit type reflector 391-57161. Appearance is identical to original, but this arrangement is much more secure against loss from theft or vibration than the original. "
I am interested to know which parts manual
you used. Mine is published September 1969, #99-0903.