Contract Source Code:

// SPDX-License-Identifier: LGPL-3.0-or-later
// Taken from: https://github.com/gnosis/safe-contracts/blob/development/contracts/proxies/GnosisSafeProxy.sol
pragma solidity ^0.7.0;

/// @title IProxy - Helper interface to access masterCopy of the Proxy on-chain
/// @author Richard Meissner - <[email protected]>
interface IProxy {
    function masterCopy() external view returns (address);
}

/// @title WalletProxy - Generic proxy contract allows to execute all transactions applying the code of a master contract.
/// @author Stefan George - <[email protected]>
/// @author Richard Meissner - <[email protected]>
contract WalletProxy {

    // masterCopy always needs to be first declared variable, to ensure that it is at the same location in the contracts to which calls are delegated.
    // To reduce deployment costs this variable is internal and needs to be retrieved via `getStorageAt`
    address internal masterCopy;

    /// @dev Constructor function sets address of master copy contract.
    /// @param _masterCopy Master copy address.
    constructor(address _masterCopy)
    {
        require(_masterCopy != address(0), "Invalid master copy address provided");
        masterCopy = _masterCopy;
    }

    /// @dev Fallback function forwards all transactions and returns all received return data.
    fallback()
    payable
    external
    {
        // solium-disable-next-line security/no-inline-assembly
        assembly {
            let _masterCopy := and(sload(0), 0xffffffffffffffffffffffffffffffffffffffff)
        // 0xa619486e == keccak("masterCopy()"). The value is right padded to 32-bytes with 0s
            if eq(calldataload(0), 0xa619486e00000000000000000000000000000000000000000000000000000000) {
                mstore(0, _masterCopy)
                return(0, 0x20)
            }
            calldatacopy(0, 0, calldatasize())
            let success := delegatecall(gas(), _masterCopy, 0, calldatasize(), 0, 0)
            returndatacopy(0, 0, returndatasize())
            if eq(success, 0) { revert(0, returndatasize()) }
            return(0, returndatasize())
        }
    }
}

Contract Source Code:

// SPDX-License-Identifier: LGPL-3.0-or-later
// Taken from: https://github.com/gnosis/safe-contracts/blob/development/contracts/proxies/GnosisSafeProxy.sol
pragma solidity ^0.7.0;

/// @title IProxy - Helper interface to access masterCopy of the Proxy on-chain
/// @author Richard Meissner - <[email protected]>
interface IProxy {
    function masterCopy() external view returns (address);
}

/// @title WalletProxy - Generic proxy contract allows to execute all transactions applying the code of a master contract.
/// @author Stefan George - <[email protected]>
/// @author Richard Meissner - <[email protected]>
contract WalletProxy {

    // masterCopy always needs to be first declared variable, to ensure that it is at the same location in the contracts to which calls are delegated.
    // To reduce deployment costs this variable is internal and needs to be retrieved via `getStorageAt`
    address internal masterCopy;

    /// @dev Constructor function sets address of master copy contract.
    /// @param _masterCopy Master copy address.
    constructor(address _masterCopy)
    {
        require(_masterCopy != address(0), "Invalid master copy address provided");
        masterCopy = _masterCopy;
    }

    /// @dev Fallback function forwards all transactions and returns all received return data.
    fallback()
    payable
    external
    {
        // solium-disable-next-line security/no-inline-assembly
        assembly {
            let _masterCopy := and(sload(0), 0xffffffffffffffffffffffffffffffffffffffff)
        // 0xa619486e == keccak("masterCopy()"). The value is right padded to 32-bytes with 0s
            if eq(calldataload(0), 0xa619486e00000000000000000000000000000000000000000000000000000000) {
                mstore(0, _masterCopy)
                return(0, 0x20)
            }
            calldatacopy(0, 0, calldatasize())
            let success := delegatecall(gas(), _masterCopy, 0, calldatasize(), 0, 0)
            returndatacopy(0, 0, returndatasize())
            if eq(success, 0) { revert(0, returndatasize()) }
            return(0, returndatasize())
        }
    }
}