Eric M is probably correct.
I recall that even quite recently at least one of the gun manufacturers (it may have been Marlin) was referring to detachible box magazines for their firearms as "clips" in their ads.
And it was only relatively recently that the formal (and correct) distinction between "clips" and "magazines" was made and came into relatively wide use by those who wished to be technically correct:
A "clip" is a device holding a number of cartridges, with which the "magazine" of a firearm may be charged.
Examples are the ordinary stripper clips which charged the magazines of many military rifles such as the Mauser series like the 98, and the "en bloc" clip of the M1 Garand rifle, where the whole clip is inserted into the magazine and ejected when the last round is fired. (I believe the Carcano-Mannlicher rifles also had this "en bloc" system of loading, and the Broomhandle Mauser (C96?) handgun originally had a non-detachable magazine loaded with stripper clips, butthey changed that to detachible magazines soon after.))
The magazine is the container or "space in the gun" which holds the cartridges ready to be fired, whether this be tubular or otherwise.
Magazines may be fixed or detachible, as in the fixed magazine of the Garand or the detachable ones of many firearms, including all modern aemiautomatic handguns. And also, of course, the AR and AK series with which we are all familiar. The M1 Carbine had stripper clips to load its detachable magazine as well.
However, Marlin (if it was indeed Marlin) probably got tired of all the correspondence relating to the incorrect usage of the term "clip" in their ads, and as a concession to many old-timers who still confuse "clip" with "magazine," decided to call it both in their ads.
I must confess that as an old-timer, I still, out of habit, call a magazine a clip every once in a while, and usually receive ten lashes for the understandable error.
So now, instead of answering letters pointing out the correct usage of the two separate terms, they'll have to answer a slew of letters inquiring as to why they are using the term clip magazine.
Unintended consequences.
Terry, 230RN
ETA jcwit beat me to it while I was composing this post.