Top of the Line, in 1978 Deep Carry
Today there is no shortage of compact, sub-compact, and even micro-compact 9mm handguns that are designed to be small enough for pocket or ankle carry. However, in the early 1970s about the best you could get in that size, barring a manually-cycled Semmerling LM4, was a .25ACP mouse gun like a Colt Junior or Browning Baby or a J-frame/Dick Special.
Then came the OMC/AMT Back Up.
I have to admit, I pocket carried this bad boy for years…
More in my column at Guns.com.
laststandonzombieisland 
I bought an OMC back in the late 70s, and liked it so much I bought another one. The second one was an AMT and, oddly enough, the magazines were not interchangeable. Of the .380s I owned, I liked the Beretta 84 the best, followed by the BackUps, and I hated the PPK/S. I got rid of all my .380s in the mid-80s, when S&W came out with the 469.
There were wallet holsters made for it (and other small guns) that had a cutout over the trigger so you could fire them without taking them from the wallet. Sometime in the 90s, ATF ruled that a gun in such a wallet holster was a “disguised weapon” and illegal. So if you have one of those holsters, get rid of it.
I still have in my possession a Raven Arms in .25 ACP as well as its “upgrade,” a Beretta Tomcat in .32 ACP. I carried the Beretta as recently as last summer when my Ruger LCP was out of commission after spending a couple of days on the bottom of a river. The LCP is back in tip-top shape so the Beretta is back in storage. With minimal summer clothing I’ll carry the LCP, but if I have the ability to conceal it properly, the S&W 45 Shield will be EDC.