Since it's 3/4" front & back and 1/2" side I'd guess it probably never had a brace to being with. Some of them older tank with thicker glass came without. You can usually find new plastic trim with the brace for that size tank for about $40 online so as OCD as I am about my tanks I think if it were me I'd probably look into just replacing it with a new one. Just make sure it fits over your thicker glass when/if you do get a new one.
Right. I found one including the glass for 104 from FTD but will search around for this $40 unit. The tank is still 18" dead nuts from front to back so i guess the extra half inch or so came out of the inside.
Thanks!