Why do some TSOP1738 have a metal cross over their window?
It is a shielding against EMI or RFI interference,
According to this article

TSOP module has an inbuilt control circuit for amplifying the coded pulses from the IR transmitter. A signal is generated when PIN photodiode receives the signals. This input signal is received by an automatic gain control (AGC). For a range of inputs, the output is fed back to AGC in order to adjust the gain to a suitable level. The signal from AGC is passed to a band pass filter to filter undesired frequencies. After this, the signal goes to a demodulator and this demodulated output drives an npn transistor. The collector output of the transistor is obtained at pin 3 of TSOP module.
Members of TSOP17xx series are sensitive to different centre frequencies of the IR spectrum.

so to avoid EMI/RFI, some of this module have the shielding, build over it