Dave;
You can still get the 20 round original magazines here (but they are used) >
Magazine, .223 Cal., 20 Round Gun Parts | 702390 | Numrich Gun Parts < might give them a call and see if they can shed some light on your question.
You can also get some parts still >
Search | Numrich Gun Parts
Don't know all the details, but I do know this much;
Markings for the Costa Mesa and Howa magazine floor plates logo varied, some had no marking, some had the Armalite crosshair, some had the Armalite leaping lion. Most all of them had the AR18 marking. The Howa magazines are less likely to have the leaping lion logo on the floor plate and more likely to have put out magazines with no logo on the floor plate. The government of Japan had a tight leash on Howa and when the thing about exporting Howa's to only non-combatant non-asian countries came about this did not include magazines but Japan did not want Japan indicated as the country of origin on the magazine.
The Sterling 20 round mags came in two versions, one used the same design as the Costa Mesa magazines. The Sterling black aluminum 20 rounders were made by a USA sub contractor manufacturer and not Armalite or Sterling. Then there was a second version of the Sterling magazine made of parkerized steel and came in 20, 30, and 40 round versions.
Color varied from black, to black with a purplish tint, to a grey color. Material for the magazines changed also, some were 6061 aluminum and some were 7075 aluminum, Sterling used steel in one version of their magazine, some were an alloy. Finish also changed and not all were parkerized. Armalite and Sterling are the only ones to have consistantly used their manufacturer location names on the floor plate ("Armalite Costa Mesa" or "Sterling England").
All of them were really AR18 magazines. The Sterling first version black magazines with the purple tint were the less durable.
Anyway, I was curious if you're a native Floridian and where you bought yours. Do you remember what you originally paid for it? That's a nice early one! The serial number on mine is S1129x. It's just a wonderful shooting gun, and I've never regretted getting it. but do regret never buying the AR18 when I was a Class III dealer. 
I have shot one in full auto though, and they are amazingly controllable! I've kicked myself for not buying a 180 that was new in the box, with all the goodies that came with it, several years ago. 
It was pricey then, and I could justify not spending the money, but have always wished I had bought it ever since. 
