revolvers are measured for barrel length from the beginning of the forcing cone to the end of the muzzle (with any muzzle devices removed). i like to use a piece of card inserted into the cylinder gap, then drop a dowel down the barrel which should stop at the dowel, mark the end, remove and measure.
Vaqueros are technically made to order. the common lengths like the 3.75" are big distributor buys (Lipsey's, RSR, et c). but you can get them from 2.25" up through 8-1/8" and several lengths in between. they are "modular" if you know what you're doing with parts swapping and have some simple hand tools. from your description, if it's rubber grips it's a New Vaquero model, if it's simulated ivory or ebony it's a Bird's Head. if it has "warning" embossed on the barrel or ejector rod cover, it's a clean-up model after production ended in 2009. if it's color-case hardened and has two grips (fancy wood or simulated ivory with rubber shooting grips) with a brass escutcheon it may be a special order or commemorative model.
a Bird's Head 4-5/8" in .45 LC with ebony grips would be a little unusual, but not impossible as a small run order from a distributor that wanted that configuration. ruger usually marks one of the grips (usually the left hand grip) with the SN of the revolver it was shipped with. grips sold separately are not marked.
Anchor-Harvey Aluminum, AF contract surplus parts with some bushmaster export parts thrown together. it's a mix-master, but it's all government surplus. note that Green Mountain was OEM for Bushmaster for a number of years and while it won't win any awards, it's a minute of man barrel and will work as a fighting rifle:
https://ar15sport.com/product/20-a2-upper-assembly-with-17-twist-barrel-upper-556-a217/
Also Anchor-Harvey or Alcoa Forge (random which you get) C7A1 uppers for federal police service contract that were swapped for complete uppers with rails and stuff. cheaply refinished, might have uneven or odd coloring, but mechanically good:
https://ar15sport.com/product/c7-stripped-upper-finished-in-black/
Rock River law enforcement contract uppers (these were replacing shotguns and bolt actions for some sheriff departments somewhere from rebuilt M16A1 or older rifles sold for cheap from the Army), new, but shows handling marks from bulk packaging and being shipped around in cardboard:
https://ar15sport.com/product/complete-rock-river-a2-upper-receiver-arpart-a2stripped/
AR15Sport is a super small shop by retired gunsmith and a couple friends. shipping might be pricey since they do USPS but they're legit and have good parts and service. excellent source if you know what you want. think Sarco but less advertising. their email signature used to be "S&W can suck a long one" back when the Clinton Crime bill was passed to give you an idea of the person running it.