Odd casting on 87 4.0 block

I helped a friend of mine change his engine in his '87 jeep cherokee. When it was on the hoist i noticed this "BT" cast into the side of it. Didnt think anything of it but once it was on the stand, i found it was cast on both sides of the block, at the end of the serial number, and even on the rear of the block next to the freeze plug behind the flexplate. I have 2 other amc 4.0 renix blocks and they dont have this on it. Just wondering if anyone knows what it means or if it means nothing?

Beginner's try?  Were the freeze plugs different too? My 4.5L motor had MX cast into the sides and only 2 freeze plugs per side

I'm thinking some of the later blocks were done in a different foundry. AMC use to use a foundry in Sarnia, Ontario Canada but it turns out to been one of dirtiest foundry s in the world. Chrysler closed it long shortly after buying AMC. Seems like a lot of the latter blocks were done in Mopar foundry some where. There are still law suits on the old Sarnia foundry I understand. It was originally an old Ford foundry before AMC bought it. LRDaum