If your barrel was made by Remington, it will have a little REM proof stamp on it, and a date code - I have a 30-06 barrel from a Remington Model 30 that is dated stamped to Jan 1926. If the barrel was chambered by Remington, it will have the correct cartridge name stamped on it. Your receiver has the serial number - when you look that up, you will get the year the receiver was made. Rem 700 actions are very popular for "custom" rifles - the receivers alone can/could be purchased or a factory rifle purchased and cannibalized for the action. Your stainless steel barrel may have been a "custom" barrel that was made up, threaded and installed on that action. I do not know, off the top of my head, what the models were that Remington made in 7mm STW. I do not know if they offered a stainless barrel option. So, it might be reasonable to assume that you have a "custom" installation, not a factory one, and you really should have a chamber cast done to verify your chamber dimensions. It may be correct, but you will not know until a cast has been done correctly and measured with a micrometer.
Most modern loading manuals are developed with pressure testing equipment - either Copper Crusher barrels or with piezometric strain gauges. They have access to SAAMI standard pressure loads to calibrate their testing machines. Older information, from 1950's, 1960's was known to be developed the same way home hand loaders now use - measuring case head expansion, measuring velocity, gauging bolt hand lift effort, "reading" primer deformities. Can read articles comparing those "home done" ways to assess pressure against actual strain gauge pressure testing - some methods work, some work in some cartridges, and some do not seem to work reliably, at all.
You mention that you were using data from Nosler, bullets from Sierra and re-formed cases. I do not know of any reason to believe anyone's data will apply to your set-up. And you are finding very different "recipes", depending on which components are selected. Wildcatters like Simpson and Jarrett may indeed have used 8mm Rem Mag brass as basis for the cartridge, but they also are deeply involved in reaming, case neck turning, chamber dimensions, etc., and all the tricks that wildcatters use. Jarrett has considerable reputation as rifle builder for "bean field" rifles - very long range deer hunting, among other things.
Nosler #7 manual, on page 385, says their 7mm STW data was developed using Nosler brand cases - at top of the chart, can see that they got 89.4 grains (weight) of water in their case, with a Nosler 175 grain Partition bullet seated to Cartridge Over All Length of 3.600". They used Federal 215 primers, and list a Start load of 68.5 grains of H1000 powder. If your cases have less internal powder room, you are going to get higher pressure. If you swap out bullets (as you are doing) or primers, you might get more or might get less pressure. Some other cartridges and rifle chambers might be more "forgiving" about swapping things - for whatever reason, your rifle thinks otherwise?
I had read of similar chambering issues with Weatherby cartridges. Factory design has a rather long "free bore" ahead of the chamber, before the bullet engages the rifling. This helps to relieve some of the initial peak pressures. Some custom rifle makers chose to chamber a Weatherby cartridge with minimal or no free bore, in pursuit of better accuracy. Those custom rifles often display significant excess pressure when firing standard factory Weatherby cartridges - the free-bore area ahead of the chamber is significant - another reason to have your chamber cast, so that you know.
And your earlier mention that you can not drop a 7mm bullet all the way through the neck of a fired case is a sure sign that there is a physical incompatibility with your brass. or your brass within that chamber - I suppose extreme pressure could "flow" case wall brass forward into the neck, but I would be looking at inside reaming or similar to make that issue go away.
