Standard '73 850s had silver barrels. '73 750s had black barrels if the brochures and ads are correct.
The brochure you refer to is the later one which really only covered the 850's and the never-seen short stroke roadster with 850-style barrel in black. There is another earlier brochure which has all the 750's and all in silver. To confuse the issue,
many ads in 1973 showed the '72 model with silver clock holders, small taillight and black barrels. I know a lot of people preferred the black barrels so it is likely shops painted the barrels to make a sale.