I have a PRS10 Rb standard mounted in a box. It's bolted to the rear of the case. No thermal compound was used. The PRS10 is similar in size and form factor to the FRS-C. The PRS10 has a steady-state power drain of 14.4W @ 25C. They don't say whether that 25C is ambient or base plate temperature. The FRS-C is rated for 12W @ 25C base plate temperature.
After running for a few hours in a 22C ambient, the base plate temperature of my PRS10 was about 45C. No further cooling is required. Even in summer, the base plate temperature won't come near the 65C limit. If I was concerned, the first thing I'd do is mount an external heatsink on the back where the PRS10 is bolted. Orient the fins vertically for best air flow. A fan isn't required. For comparison, The Efratom PRFS Rb frequency standard contains an FRS standard. It has no fan. On the rear, there's a heatsink with a small number of thin fins. The unit is rated for operation in a 50C ambient.
Towger, are you sure that heatsink is original? The only way you'd need such a large heatsink is if you were running in a very high ambient temperature. The fins hanging out past the ends is also suspicious. In your pictures I can't see any holes, clips, etc. How would it be mounted originally?
Ed