WIFGBP
WIF / Pound sterling
crypto

I would think you are looking for a custom encoding, rather than a custom encryption. There were certainly people doing a lot of non-standard things at the time, because there were not many standards yet. Often it was a tweak of an existing standard. That 5 still has me think WIF.

If the second character is an H, J or that means the hex encoding would start with the byte "80"- which is the flag for mainnet in the uncompressed WIF format. That is followed by 32 bytes and a checksum. The checksum is "unnecessary", so to save characters that needed written down, they may have dropped it. 33 bytes can be 45 character when encoded in base58.

I would think you are looking for a custom encoding, rather than a custom encryption. There were certainly people doing a lot of non-standard things at the time, because there were not many standards yet. Often it was a tweak of an existing standard. That 5 still has me think WIF.

If the second character is an H, J or that means the hex encoding would start with the byte "80"- which is the flag for mainnet in the uncompressed WIF format. That is followed by 32 bytes and a checksum. The checksum is "unnecessary", so to save characters that needed written down, they may have dropped it. 33 bytes can be 45 character when encoded in base58.

WIF private keys start with a 5 and ere ued around that time period, but they are 52 characters, maybe truncated?
Does it appear to be base58?