Or maybe someone had already pawned it! /have read waaaaay too much Agatha Christie, half of which has this plot

Good point. After all, F1 had left the country in debt for 20 Million Reichstaler, his administration was famously corrupt, and secret jewlry sales would fit right in. FW didn't buy any new jewelry - whatever Ulrike got in his lifetime was from what was left from her grandfather's day. Fritz might have bought her something new on the occasion of her wedding, but....