I forgot to post this some years ago. This reference is to the JBL H91/2307 horns. Evidently JBL changed out the castings at sometime in the series production. I have not seen it on the longer H92/2312 horns.
The castings on the earlier horns was much thicker. Upon weighing several horns it is almost 1 pound.
The highest serial number of the thick cast ones I have is #17104. Its weight being 2pds 9.8oz. this was the lightest one of my samples. The thin cast H91 with ribs lowest serial #17139 at 1pd 10.6oz also a 2307 horn 1pd 10.4oz serial number #38970. I have not got to my other 2307 horns to see if there is any thick cast ones and what the serial numbers would be. The weights varied by 2 to 3 ounces on all samples.