Received a M1640 in the shop today with the above complaint. I prepared some primed brass and tested it. Starting at 1 o'clock bolt position I cocked and fired it all the way to about 5 degrees from being closed. At that point it did fire. This action has a single stage trigger. Cocks on open. Has 98 style cocking piece, bolt shroud, and safety lug. FP protrusion is .045. FP is 96 style with no safety tabs. There is no provision to disable the trigger if the bolt is not fully closed. It depends entirely on the cocking cam slowing the FP fall to prevent a discharge. Any suggestions on how to approach this?

That rifle is operating as designed, with the combination of parts that you have.
On the 96 series, with no FP safety interlock, it relied on a plunger rising into a slot into the bolt to prevent early firing. Also the retracting cam does contribute.
When they went to the 98 the first ones were made with no FP safety, but also with no sear plunger either. And has the cocking cam that prevents early firing. Yours is that, and it will act as yours does.
However, I just checked one with the normal parts and it will fire at 5 degrees open; there is a lot of clearance in the cocking cam/sear interface. Five degrees is not much.
No modern design uses anything but the cocking cam to prevent FP protrusion; I have never tested one to see exactly when the FP will protrude enough to fire a primer but I am sure it is only after they are locked enough to prevent issue; otherwise we would hear about them; as many rifle owners are not all that savvy about mechanical devices like rifles. Or scissors.
I would put a new FP into it because your customer is not confident.. If it was mine I would just shoot it.

Thanks "d". Good info. The client also mentioned he thinks he got a puff of gas in the face but i kinda doubt that. Like you said that tiny 5 degrees couldn't account for more than a thou or two. The lugs are on the flats by then. Yeah, you have to do something to the rifle or the customer will balk. Maybe I should just tell him it is a design fault and offer to buy it off him for parts.

Yeah; it is a good action for a custom build!
And yes, no way he got a puff of gas due to the locking lugs.