I believe you, but just curious if you could provide any examples specifically? To my aging eyes on my crappy 42" 1080P TV the photos look identical other than size.
Mea Culpa! (Latin for 'I fuct up' ).
For sure there was one that had a dimple machined in its blind end and a small amount of machining somewhere else on the same casting and looked slightly less sand cast - more finished. And I can't find it on any of the options now. It was one of the options where there were plenty photos and I'd swear it was the ACDelco one but when I look now I agree fully, the two items look identical and probably are the same photos.
In my defence! - Rock Auto change their listings on whim. Eg, Thursday night, earliest ETA for the Raybestos MC showed as 10th Dec. When I came to order it on Friday that had changed to the 9th.
Does it match any of the "Alternate/OEM Part Numbers" down at the bottom of the page on those listings? Specifically I'm curious about 18029968. From what I can tell according to Nemiga Parts that should be the OEM GM master cylinder part number for your truck. Discontinued, unfortunately.
Part number on mine is 18012985 (assuming I've read it correctly with a mirror) and '389 75 80' is also there but stamped - not cast. Couldn't find that number on any of RA's listings but when I hovered over the 'MC390257' Raybestos part number a pop-up had a list of vehicles it fitted and mine was included. That, and that I'd accessed that page by entering all the vehicle details kind of throws it back to RA if it's wrong. I just had to get it ordered as needed ASAP.
At a guess, if it is wrong it will be internal dimensions. Presumably I will have a MC I can fit and a usable truck again until RA correct their mistake.
Kind of regret not changing the MC earlier - I got blindsided by someone suggesting that the PAS pump could be a cause. One of those things that had to run a little longer to fail a bit more and offer up a more certain diagnosis. The braking fading in the mid part of the pedal travel leaves me in little doubt that the MC is done. And also points to the danger of bleeding by the pedal stroke method. Definitely looking to do it differently in future. To that end, I want a good look at the bottom of the reservoir where the fluid leaves to see what can be coupled there to pressurise without having to pressurise the reservoir body. Going by what I've read, there's a fitting for that purpose.