Like most older companies, Lawrence didn't make shoulder holsters to fit a specific gun.
They made revolver models and automatic models. The main difference between the two is the revolver holster had a spring that was more rounded to fit a cylinder, and the auto models had a flatter spring to fit auto slides.
As for the rest, they usually made a model for small revolvers, one for medium and one for large, with the length changed for different barrel lengths.
In other words, if you have a revolver shoulder holster and it fits a 4" medium frame revolver like a Colt Official Police or a S&W "K" frame, it would fit any revolver of a similar frame size and barrel length.
The numbers were simply stock numbers so sellers could identify the size and barrel length of a generic gun size and barrel.
Assuming size 579 is for a medium frame 4" barrel revolver, it would fit any Colt, S&W, or other similar gun.
Unless someone has a Lawrence model chart, just insert a gun into the holster. If it fits, that what it was made to fit.
Thanks dfariswheel for the reply- I believe you are referring to Lawrence Nos 6, 7 & 9 all of which have spring clips. The No 8 that I have is a shoulder holster with a standard holster with a strap. This one fits my 6 in Python as I thought - BUT the snap on the strap is about 2 inches too short too close. I have a similar No 8C (C=chamois lined) that fits my Gov't 45 Auto and is marked 615. I know, some pictures would help - I'll send some later. I thought that there were some codes used by Lawrence to ID different models? Thanks, again.
There were different codes used by Lawrence to ID what guns a holster would fit.
In your case, I'd suspect the holster was made to fit a revolver without an adjustable rear sight or a target hammer.
If you post this question on one of the other sites like The High Road or The Firing Line there are people there who I recall as having old Lawrence catalogs and can probably answer your question as to the stock numbers.
I keep a photocopy of the Lawrence holster chart in my gun show backpack. Many sellers have them on tables either unidentified or misidentified. The 1C prefix holsters are their Challenger line which are a lighter weight leather and more plain, mainly to compete with brands like Hunter. The regular line are of course very well made and starting to be collectible like the Heisers but not yet up to those price levels. A couple of years ago I bought 3 Lawrence holsters from a gun show seller who didn't know what they fit for a total of $45. None were for guns I owned so I cleaned them up and put them on ebay, properly identified, and they brought something like $150 at auction.