you need to ask them!
It may be a standard manufacturing trimming technique to laser ablate metal off of the surface of the SAW filter. Precision resistors are commonly trimmed this way.
The question to ask them...is this a standard method of theirs, are they controlled for manufacturing processes (such as ISO certified, or provide SAW filters to Mil-Std quality), and if these SAW's were manufactured in accordance with all the required steps and quality controlled inspections and tests.
If so, get them to put that in writing.
You might get a few saw filters, put them in a test fixture, test, then burn them in at 125 deg C for a week, and compare test data before and after. Any shift in RF performance, especially increased insertion loss, will tell you if the process was not a stable one.