You also need to be SURE it's really a factory engraved gun.
The only way to be 100% sure is to buy a Colt Historical Letter from Colt's archive service.
Price is around $75. to $100 dollars but it'll plainly state whether the gun was a Colt Custom Shop gun, or something someone SAYS is a factory gun.
http://coltsmfg.com/cmci/historical.asp
When dealing with higher dollars, take no chances.
While it's very possible a non-factory engraved gun can be just as valuable, that all depends on WHO did the engraving.
A top engraver's work brings top dollar.
Another major factor is what level is the engraving. Colt engraving runs from "A" to "D". "A" level is about 1/4 coverage, with "D" level being about 100% coverage.
The higher the level, the higher the price.
SA035xx was made in 1978.
The "SA" series started that year, and began at SA01001.
The number ran to SA14808 by the end of the year.
Again, just because someone SAYS it's a factory engraved gun is not PROOF.
Also, keep in mind that box labels are routinely faked up these days, so just because the label says it's a Custom Shop job, is no guarantee.
ONLY a factory letter is proof.