60 hp Water pump gaskets

[Avanti]

60 hp 4 stroke 1.83:1 lower unit
Ser no. 1C558025
2019 257 hours

Background: I did the water pump replacement recently, for the first time.
Replaced parts in the 300 hour kit.
Impeller looked like new with a slight set.
I monitor the water temperature with mobile vessel view. Highest temperatures low 130's, usually high 120's.
Thermostat looked good, I could see it open on vessel view on warmup.
When dismantling the water pump, it looked like parts of both gaskets were missing, I think towards the rear.

Question: Could the gasket be partially blown out from operating the engine in idle on the hose/muffs, due to excessive hose pressure ?

Unrelated: Look's like the wrong thermostat was in the kit. I don't know if that's been an issue. It is different physically and it seems to be open when I ran it.

[EuropeanAM]

What Part Number kit did you use (and from where)?

The thermostat in the CORRECT kit is the correct one for your engine.

8M0113483 is the correct kit- and the thermostat has changed (superseded), so there may be some differences in APPEARANCE. Engine should, however, reach 120+ degrees at idle without any significant difficulty.
Water Pump gaskets seldom blow from idle operation or operating the engine on muffs. Usually occurs due to improper torque on the screws, or sudden changes in pressure (such as rough water operation where the engine is bounding in and out of the water).

[Avanti]

Thanks.
The 300 hour kit part number was 8M0113483 purchased from Boats.net.
The part of the thermostat opposite the gasket has a narrower diameter than the old one.

[EuropeanAM]

Normal on thermostat.