Here is the official modification to expand both the TX and RX capabilities of the Icom ID-51:
RX frequency expansion
A BAND
VHF: Expanded frequency range: 137.000‐174.000MHz. (FM/FMN/DV)
UHF: Expanded frequency range: 380.000‐479.000MHz. (FM/FMN/DV)
B BAND
VHF: Expanded frequency range: 108.000‐136.995MHz. (AM), 137.000‐174.000MHz. (FM)
UHF: Expanded frequency range: 380.000‐479.000MHz. (FM)
Remove D9 on Logic board.
TX frequency expansion
VHF: Expanded frequency range: 137.000‐174.000MHz. (FM/FMN/DV)
UHF: Expanded frequency range: 400.000‐479.000MHz. (FM/FMN/DV)
Remove D11 on Logic board for VHF band.
Remove D10 on Logic board for UHF band.
See the attached PDF for locations of the referenced diodes.
RX frequency expansion
A BAND
VHF: Expanded frequency range: 137.000‐174.000MHz. (FM/FMN/DV)
UHF: Expanded frequency range: 380.000‐479.000MHz. (FM/FMN/DV)
B BAND
VHF: Expanded frequency range: 108.000‐136.995MHz. (AM), 137.000‐174.000MHz. (FM)
UHF: Expanded frequency range: 380.000‐479.000MHz. (FM)
Remove D9 on Logic board.
TX frequency expansion
VHF: Expanded frequency range: 137.000‐174.000MHz. (FM/FMN/DV)
UHF: Expanded frequency range: 400.000‐479.000MHz. (FM/FMN/DV)
Remove D11 on Logic board for VHF band.
Remove D10 on Logic board for UHF band.
See the attached PDF for locations of the referenced diodes.