I have uploaded pictures
of the base assemblies of 2 versions of the Sigma 5/8. One is after the Antenna Specialist (A/S) buy-out and the other is an original Avanti product.
You will notice that Antenna Specialist, with their deeper resources, mechanically upgraded the antenna from it's earlier design. Pay special attention to the two hub assemblies. The newer one (with cast #1007) is THICKER around the hole for the base tube. The older hub is cast #MR101.
The base tube was also lengthened and double walled. The inner tube of the A/S part has the same diameter (1 5/8 x .058 wall) as the older base tube. The outer tube is now 1 3/4 x 18"
The feedpoints are completely different. A/S added a coil which made the Sigma 5/8 DC grounded for the first time! They also used 3 bolts around the bottom to secure it to the base tube. The length of the 10 guage wire (printed "Phelps Dodge" on the insulation) was also increased when the coil was added. You can also see the longer fiberglass tube. The top section of the main radiator was also shorten approx 6" to bring the electrical length of the antenna back to 5/8.
From the center conductor of the coax jack, the signal goes up the orange wire to the top aluminum block. The bottom of the loop attaches here. The signal travels around the loop an into the base of the main radiator. Inside the fiberglass tube, there is a 1" gap between the base of the main radiator and the aluminum block. This is Avanti's elevated feed system I've spoken of in the past. The main radiator is NEVER surrounded by any metal at ground potential (unlike the Penetrator).
The base of the main element has an aluminum boss pressed in and tapped to accept the bolt that secures the top of the loop. The bolt head you see INSIDE the tube is a plug to keep the water out of the feedpoint. There is a small hole in the tubing just above this plug to let the water out.
Antenna Specialist has machines that punched the slots into the tubing. Avanti used a band saw. The last pictures shows a couple of original Sigma IV antennas, some new Alcoa 6061-T6 aluminum (.058 wall x 12' each, from 1/2" to 1 3/4"), and some old ham radio antennas I've remove from towers for other people. This is only abou 1/10th of the aluminum I have at home.
I've done my best to illustrate the two versions of the Sigma 5/8. If you have any questions, please ask!