i'll try to help...the picture i found in a fjestad blue book of a pre 1912 slide has the horsey behind the cocking serrations, left slide side. can't tell from your jpg above, as it stops before same.
below, from:
http://www.model1911a1.com/
..."Also of note are the Type 4 slide markings. The Rampant Colt has been moved to mid-slide between the patent and address lines and they are now stamped using a block style lettering font which continued thru approximately serial number C125000. Less noticeable is the reshaping of the recoil spring housing."...
so, if your slide has the "rampant colt" on left, rear part of slide in that is missing in your jpg, it is likely mfgd from 1912-1919. the one in his his posting at that address/web site was made in 1919. looking into the reshaped recoil spring housing will be another clue to follow, as is the explanation of what types 1-3 looked like.
go to the address posted above, he has many nice old pistols for sale, WITH JPGs. click on "pre war commercial" at lower left of homepage....
also see:
http://www.milsurps.com/showthread.php?t=14455.
i did a google search of "1911 colt commercial slides + for sale" and on the first two pages got NO listings. got one guy looking for a military slide, but this is likely not what he wants, he's cobbling together a military version. so you can see how in depth your search can get.
this is the sort of web searching that will yield the answers you seek.
i would suggest avoiding going to "the local gun store" seeking info about this slide. especially if you want to remain on friendly terms with the owner. unless he is a hardcore collector, he will not know. in that case he may try to low ball an estimate, hoping to scam you. or just say whatever comes to mind in order to protect his authority figure delusions.
gunnie
EDIT: my bad. i see the horsey in the middle of the scroll marks of jpg. make that a post 1919 mfg date.