The guys wearing white helmets, jodhpurs, and boots are riding two wheelers. In the photos and films, you'll see that the guys on three-wheelers are all wearing white hats rather than helmets. The patrol division guys are the ones with green shoulder tabs; traffic division officers have red tabs.
So the officer in the foreground of Willis 8 would be a traffic division officer (red tabs)?
I know what you mean about Barnett's testimony. Something is odd in there about the Sergeant. But the channel 2 recording is what it is.
It's hard to know what to think.
It seems unlikely Barnett wouldn't know his own sergeant.
It seems equally unlikely Howard would be around the TSBD if his assignment was near the Trade Mart.
A scenario where Howard rushes away from the scene of the assassination to pick up some "No Parking" signs elsewhere isn't going to fly.
The thing is, if you compare Barnett's description of Brennan's approach, Brennan doesn't mention a sergeant. However, Brennan does mention that Barnett led him to Forrest Sorrels at the front of the Depository. Sorrels returned to the TSBD about 12:45. Barnett leading Brennan to Sorrels would also indicate that Barnett's testimony covers a much longer span of time than he remembered.
Barnett said that all three of them rushed to the TSBD steps and, more importantly, he kept Brennan with him on the steps.
This seems to be confirmed by various Murray pics:

In the above pic Brennan (arrow) leans against the wall of TSBD entrance.

Brennan now seems to be moving down the steps
share imageIf you look closely in the above pic you can make out (who I assume to be) Brennan and Sawyer in Sawyer's car
Sorrels supports Barnett's testimony with his own:
"I asked for the manager, and I was directed to Mr. Truly. He was standing there.
I went up and identified myself to him. I said, "I want to get a stenographer, and we would like to have you put down the names and addresses of every employee of the building, in the building."
And
I then walked on out the front door and asked, "Did anyone here see anything?"
And someone pointed to Mr. Brennan."
It appears Brennan was waiting on the steps when Sorrels arrived.
One thing I have just noticed (this is a real testament to my powers of observation), Brennan cannot be seen in Willis 8 or Skaggs 2.