Ive had this m head for about 20 years laying around with several other m heads. And i noticed that the casting is different. Theres 3 extra bolt holes through the front that are threaded with allen head bolts. Also theres no serial number on it. Ive looked through lots of images on google search for m heads. But never saw one like this.
I have noticed that bridgeport did make a few oddball heads for different applications I suppose. Im wondering if anyone can tell me what this head was used on and why it has the addition to the casting.
Im gonna try and figure out how to get photos on here. If not, I'll put on a link. So bear with me...

https://flic.kr/p/2bmmQ58